[PATCH] Documentation update for relay interface
- From: Tom Zanussi <zanussi@xxxxxxxxxx>
- Date: Wed, 23 Aug 2006 16:31:45 -0500
From: Tom Zanussi <zanussi@xxxxxxxxxx>
Hi,
Here's updated documentation for the relay interface, rewritten to
match the relayfs->relay changes. It also moves relayfs.txt to
relay.txt in the process.
It includes the changes to relayfs.txt previously posted by Randy
Dunlap, thanks for those.
The relay-apps examples have also been updated to match, and can be
found on the sourceforge relayfs website.
Tom
Signed-off-by: Tom Zanussi <zanussi@xxxxxxxxxx>
Documentation/filesystems/relayfs.txt | 442 -------------------------------
b/Documentation/filesystems/00-INDEX | 4
b/Documentation/filesystems/relay.txt | 479 ++++++++++++++++++++++++++++++++++
3 files changed, 481 insertions(+), 444 deletions(-)
diff --git a/Documentation/filesystems/00-INDEX b/Documentation/filesystems/00-INDEX
index 66fdc07..16dec61 100644
--- a/Documentation/filesystems/00-INDEX
+++ b/Documentation/filesystems/00-INDEX
@@ -62,8 +62,8 @@ ramfs-rootfs-initramfs.txt
- info on the 'in memory' filesystems ramfs, rootfs and initramfs.
reiser4.txt
- info on the Reiser4 filesystem based on dancing tree algorithms.
-relayfs.txt
- - info on relayfs, for efficient streaming from kernel to user space.
+relay.txt
+ - info on relay, for efficient streaming from kernel to user space.
romfs.txt
- description of the ROMFS filesystem.
smbfs.txt
diff --git a/Documentation/filesystems/relay.txt b/Documentation/filesystems/relay.txt
new file mode 100644
index 0000000..d6788da
--- /dev/null
+++ b/Documentation/filesystems/relay.txt
@@ -0,0 +1,479 @@
+relay interface (formerly relayfs)
+==================================
+
+The relay interface provides a means for kernel applications to
+efficiently log and transfer large quantities of data from the kernel
+to userspace via user-defined 'relay channels'.
+
+A 'relay channel' is a kernel->user data relay mechanism implemented
+as a set of per-cpu kernel buffers ('channel buffers'), each
+represented as a regular file ('relay file') in user space. Kernel
+clients write into the channel buffers using efficient write
+functions; these automatically log into the current cpu's channel
+buffer. User space applications mmap() or read() from the relay files
+and retrieve the data as it becomes available. The relay files
+themselves are files created in a host filesystem, e.g. debugfs, and
+are associated with the channel buffers using the API described below.
+
+The format of the data logged into the channel buffers is completely
+up to the kernel client; the relay interface does however provide
+hooks which allow kernel clients to impose some structure on the
+buffer data. The relay interface doesn't implement any form of data
+filtering - this also is left to the kernel client. The purpose is to
+keep things as simple as possible.
+
+This document provides an overview of the relay interface API. The
+details of the function parameters are documented along with the
+functions in the relay interface code - please see that for details.
+
+Semantics
+=========
+
+Each relay channel has one buffer per CPU, each buffer has one or more
+sub-buffers. Messages are written to the first sub-buffer until it is
+too full to contain a new message, in which case it it is written to
+the next (if available). Messages are never split across sub-buffers.
+At this point, userspace can be notified so it empties the first
+sub-buffer, while the kernel continues writing to the next.
+
+When notified that a sub-buffer is full, the kernel knows how many
+bytes of it are padding i.e. unused space occurring because a complete
+message couldn't fit into a sub-buffer. Userspace can use this
+knowledge to copy only valid data.
+
+After copying it, userspace can notify the kernel that a sub-buffer
+has been consumed.
+
+A relay channel can operate in a mode where it will overwrite data not
+yet collected by userspace, and not wait for it to be consumed.
+
+The relay channel itself does not provide for communication of such
+data between userspace and kernel, allowing the kernel side to remain
+simple and not impose a single interface on userspace. It does
+provide a set of examples and a separate helper though, described
+below.
+
+The read() interface both removes padding and internally consumes the
+read sub-buffers; thus in cases where read(2) is being used to drain
+the channel buffers, special-purpose communication between kernel and
+user isn't necessary for basic operation.
+
+One of the major goals of the relay interface is to provide a low
+overhead mechanism for conveying kernel data to userspace. While the
+read() interface is easy to use, it's not as efficient as the mmap()
+approach; the example code attempts to make the tradeoff between the
+two approaches as small as possible.
+
+klog and relay-apps example code
+================================
+
+The relay interface itself is ready to use, but to make things easier,
+a couple simple utility functions and a set of examples are provided.
+
+The relay-apps example tarball, available on the relay sourceforge
+site, contains a set of self-contained examples, each consisting of a
+pair of .c files containing boilerplate code for each of the user and
+kernel sides of a relay application. When combined these two sets of
+boilerplate code provide glue to easily stream data to disk, without
+having to bother with mundane housekeeping chores.
+
+The 'klog debugging functions' patch (klog.patch in the relay-apps
+tarball) provides a couple of high-level logging functions to the
+kernel which allow writing formatted text or raw data to a channel,
+regardless of whether a channel to write into exists or not, or even
+whether the relay interface is compiled into the kernel or not. These
+functions allow you to put unconditional 'trace' statements anywhere
+in the kernel or kernel modules; only when there is a 'klog handler'
+registered will data actually be logged (see the klog and kleak
+examples for details).
+
+It is of course possible to use the relay interface from scratch,
+i.e. without using any of the relay-apps example code or klog, but
+you'll have to implement communication between userspace and kernel,
+allowing both to convey the state of buffers (full, empty, amount of
+padding). The read() interface both removes padding and internally
+consumes the read sub-buffers; thus in cases where read(2) is being
+used to drain the channel buffers, special-purpose communication
+between kernel and user isn't necessary for basic operation. Things
+such as buffer-full conditions would still need to be communicated via
+some channel though.
+
+klog and the relay-apps examples can be found in the relay-apps
+tarball on http://relayfs.sourceforge.net
+
+The relay interface user space API
+==================================
+
+The relay interface implements basic file operations for user space
+access to relay channel buffer data. Here are the file operations
+that are available and some comments regarding their behavior:
+
+open() enables user to open an _existing_ channel buffer.
+
+mmap() results in channel buffer being mapped into the caller's
+ memory space. Note that you can't do a partial mmap - you
+ must map the entire file, which is NRBUF * SUBBUFSIZE.
+
+read() read the contents of a channel buffer. The bytes read are
+ 'consumed' by the reader, i.e. they won't be available
+ again to subsequent reads. If the channel is being used
+ in no-overwrite mode (the default), it can be read at any
+ time even if there's an active kernel writer. If the
+ channel is being used in overwrite mode and there are
+ active channel writers, results may be unpredictable -
+ users should make sure that all logging to the channel has
+ ended before using read() with overwrite mode. Sub-buffer
+ padding is automatically removed and will not be seen by
+ the reader.
+
+sendfile() transfer data from a channel buffer to an output file
+ descriptor. Sub-buffer padding is automatically removed
+ and will not be seen by the reader.
+
+poll() POLLIN/POLLRDNORM/POLLERR supported. User applications are
+ notified when sub-buffer boundaries are crossed.
+
+close() decrements the channel buffer's refcount. When the refcount
+ reaches 0, i.e. when no process or kernel client has the
+ buffer open, the channel buffer is freed.
+
+In order for a user application to make use of relay files, the
+host filesystem must be mounted. For example,
+
+ mount -t debugfs debugfs /debug
+
+NOTE: the host filesystem doesn't need to be mounted for kernel
+ clients to create or use channels - it only needs to be
+ mounted when user space applications need access to the buffer
+ data.
+
+
+The relay interface kernel API
+==============================
+
+Here's a summary of the API the relay interface provides to in-kernel clients:
+
+TBD(curr. line MT:/API/)
+ channel management functions:
+
+ relay_open(base_filename, parent, subbuf_size, n_subbufs,
+ callbacks)
+ relay_close(chan)
+ relay_flush(chan)
+ relay_reset(chan)
+
+ channel management typically called on instigation of userspace:
+
+ relay_subbufs_consumed(chan, cpu, subbufs_consumed)
+
+ write functions:
+
+ relay_write(chan, data, length)
+ __relay_write(chan, data, length)
+ relay_reserve(chan, length)
+
+ callbacks:
+
+ subbuf_start(buf, subbuf, prev_subbuf, prev_padding)
+ buf_mapped(buf, filp)
+ buf_unmapped(buf, filp)
+ create_buf_file(filename, parent, mode, buf, is_global)
+ remove_buf_file(dentry)
+
+ helper functions:
+
+ relay_buf_full(buf)
+ subbuf_start_reserve(buf, length)
+
+
+Creating a channel
+------------------
+
+relay_open() is used to create a channel, along with its per-cpu
+channel buffers. Each channel buffer will have an associated file
+created for it in the host filesystem, which can be and mmapped or
+read from in user space. The files are named basename0...basenameN-1
+where N is the number of online cpus, and by default will be created
+in the root of the filesystem (if the parent param is NULL). If you
+want a directory structure to contain your relay files, you should
+create it using the host filesystem's directory creation function,
+e.g. debugfs_create_dir(), and pass the parent directory to
+relay_open(). Users are responsible for cleaning up any directory
+structure they create, when the channel is closed - again the host
+filesystem's directory removal functions should be used for that,
+e.g. debugfs_remove().
+
+In order for a channel to be created and the host filesystem's files
+associated with its channel buffers, the user must provide definitions
+for two callback functions, create_buf_file() and remove_buf_file().
+create_buf_file() is called once for each per-cpu buffer from
+relay_open() and allows the user to create the file which will be used
+to represent the corresponding channel buffer. The callback should
+return the dentry of the file created to represent the channel buffer.
+remove_buf_file() must also be defined; it's responsible for deleting
+the file(s) created in create_buf_file() and is called during
+relay_close().
+
+Here are some typical definitions for these callbacks, in this case
+using debugfs:
+
+/*
+ * create_buf_file() callback. Creates relay file in debugfs.
+ */
+static struct dentry *create_buf_file_handler(const char *filename,
+ struct dentry *parent,
+ int mode,
+ struct rchan_buf *buf,
+ int *is_global)
+{
+ return debugfs_create_file(filename, mode, parent, buf,
+ &relay_file_operations);
+}
+
+/*
+ * remove_buf_file() callback. Removes relay file from debugfs.
+ */
+static int remove_buf_file_handler(struct dentry *dentry)
+{
+ debugfs_remove(dentry);
+
+ return 0;
+}
+
+/*
+ * relay interface callbacks
+ */
+static struct rchan_callbacks relay_callbacks =
+{
+ .create_buf_file = create_buf_file_handler,
+ .remove_buf_file = remove_buf_file_handler,
+};
+
+And an example relay_open() invocation using them:
+
+ chan = relay_open("cpu", NULL, SUBBUF_SIZE, N_SUBBUFS, &relay_callbacks);
+
+If the create_buf_file() callback fails, or isn't defined, channel
+creation and thus relay_open() will fail.
+
+The total size of each per-cpu buffer is calculated by multiplying the
+number of sub-buffers by the sub-buffer size passed into relay_open().
+The idea behind sub-buffers is that they're basically an extension of
+double-buffering to N buffers, and they also allow applications to
+easily implement random-access-on-buffer-boundary schemes, which can
+be important for some high-volume applications. The number and size
+of sub-buffers is completely dependent on the application and even for
+the same application, different conditions will warrant different
+values for these parameters at different times. Typically, the right
+values to use are best decided after some experimentation; in general,
+though, it's safe to assume that having only 1 sub-buffer is a bad
+idea - you're guaranteed to either overwrite data or lose events
+depending on the channel mode being used.
+
+The create_buf_file() implementation can also be defined in such a way
+as to allow the creation of a single 'global' buffer instead of the
+default per-cpu set. This can be useful for applications interested
+mainly in seeing the relative ordering of system-wide events without
+the need to bother with saving explicit timestamps for the purpose of
+merging/sorting per-cpu files in a postprocessing step.
+
+To have relay_open() create a global buffer, the create_buf_file()
+implementation should set the value of the is_global outparam to a
+non-zero value in addition to creating the file that will be used to
+represent the single buffer. In the case of a global buffer,
+create_buf_file() and remove_buf_file() will be called only once. The
+normal channel-writing functions, e.g. relay_write(), can still be
+used - writes from any cpu will transparently end up in the global
+buffer - but since it is a global buffer, callers should make sure
+they use the proper locking for such a buffer, either by wrapping
+writes in a spinlock, or by copying a write function from relay.h and
+creating a local version that internally does the proper locking.
+
+Channel 'modes'
+---------------
+
+relay channels can be used in either of two modes - 'overwrite' or
+'no-overwrite'. The mode is entirely determined by the implementation
+of the subbuf_start() callback, as described below. The default if no
+subbuf_start() callback is defined is 'no-overwrite' mode. If the
+default mode suits your needs, and you plan to use the read()
+interface to retrieve channel data, you can ignore the details of this
+section, as it pertains mainly to mmap() implementations.
+
+In 'overwrite' mode, also known as 'flight recorder' mode, writes
+continuously cycle around the buffer and will never fail, but will
+unconditionally overwrite old data regardless of whether it's actually
+been consumed. In no-overwrite mode, writes will fail, i.e. data will
+be lost, if the number of unconsumed sub-buffers equals the total
+number of sub-buffers in the channel. It should be clear that if
+there is no consumer or if the consumer can't consume sub-buffers fast
+enough, data will be lost in either case; the only difference is
+whether data is lost from the beginning or the end of a buffer.
+
+As explained above, a relay channel is made of up one or more
+per-cpu channel buffers, each implemented as a circular buffer
+subdivided into one or more sub-buffers. Messages are written into
+the current sub-buffer of the channel's current per-cpu buffer via the
+write functions described below. Whenever a message can't fit into
+the current sub-buffer, because there's no room left for it, the
+client is notified via the subbuf_start() callback that a switch to a
+new sub-buffer is about to occur. The client uses this callback to 1)
+initialize the next sub-buffer if appropriate 2) finalize the previous
+sub-buffer if appropriate and 3) return a boolean value indicating
+whether or not to actually move on to the next sub-buffer.
+
+To implement 'no-overwrite' mode, the userspace client would provide
+an implementation of the subbuf_start() callback something like the
+following:
+
+static int subbuf_start(struct rchan_buf *buf,
+ void *subbuf,
+ void *prev_subbuf,
+ unsigned int prev_padding)
+{
+ if (prev_subbuf)
+ *((unsigned *)prev_subbuf) = prev_padding;
+
+ if (relay_buf_full(buf))
+ return 0;
+
+ subbuf_start_reserve(buf, sizeof(unsigned int));
+
+ return 1;
+}
+
+If the current buffer is full, i.e. all sub-buffers remain unconsumed,
+the callback returns 0 to indicate that the buffer switch should not
+occur yet, i.e. until the consumer has had a chance to read the
+current set of ready sub-buffers. For the relay_buf_full() function
+to make sense, the consumer is reponsible for notifying the relay
+interface when sub-buffers have been consumed via
+relay_subbufs_consumed(). Any subsequent attempts to write into the
+buffer will again invoke the subbuf_start() callback with the same
+parameters; only when the consumer has consumed one or more of the
+ready sub-buffers will relay_buf_full() return 0, in which case the
+buffer switch can continue.
+
+The implementation of the subbuf_start() callback for 'overwrite' mode
+would be very similar:
+
+static int subbuf_start(struct rchan_buf *buf,
+ void *subbuf,
+ void *prev_subbuf,
+ unsigned int prev_padding)
+{
+ if (prev_subbuf)
+ *((unsigned *)prev_subbuf) = prev_padding;
+
+ subbuf_start_reserve(buf, sizeof(unsigned int));
+
+ return 1;
+}
+
+In this case, the relay_buf_full() check is meaningless and the
+callback always returns 1, causing the buffer switch to occur
+unconditionally. It's also meaningless for the client to use the
+relay_subbufs_consumed() function in this mode, as it's never
+consulted.
+
+The default subbuf_start() implementation, used if the client doesn't
+define any callbacks, or doesn't define the subbuf_start() callback,
+implements the simplest possible 'no-overwrite' mode, i.e. it does
+nothing but return 0.
+
+Header information can be reserved at the beginning of each sub-buffer
+by calling the subbuf_start_reserve() helper function from within the
+subbuf_start() callback. This reserved area can be used to store
+whatever information the client wants. In the example above, room is
+reserved in each sub-buffer to store the padding count for that
+sub-buffer. This is filled in for the previous sub-buffer in the
+subbuf_start() implementation; the padding value for the previous
+sub-buffer is passed into the subbuf_start() callback along with a
+pointer to the previous sub-buffer, since the padding value isn't
+known until a sub-buffer is filled. The subbuf_start() callback is
+also called for the first sub-buffer when the channel is opened, to
+give the client a chance to reserve space in it. In this case the
+previous sub-buffer pointer passed into the callback will be NULL, so
+the client should check the value of the prev_subbuf pointer before
+writing into the previous sub-buffer.
+
+Writing to a channel
+--------------------
+
+Kernel clients write data into the current cpu's channel buffer using
+relay_write() or __relay_write(). relay_write() is the main logging
+function - it uses local_irqsave() to protect the buffer and should be
+used if you might be logging from interrupt context. If you know
+you'll never be logging from interrupt context, you can use
+__relay_write(), which only disables preemption. These functions
+don't return a value, so you can't determine whether or not they
+failed - the assumption is that you wouldn't want to check a return
+value in the fast logging path anyway, and that they'll always succeed
+unless the buffer is full and no-overwrite mode is being used, in
+which case you can detect a failed write in the subbuf_start()
+callback by calling the relay_buf_full() helper function.
+
+relay_reserve() is used to reserve a slot in a channel buffer which
+can be written to later. This would typically be used in applications
+that need to write directly into a channel buffer without having to
+stage data in a temporary buffer beforehand. Because the actual write
+may not happen immediately after the slot is reserved, applications
+using relay_reserve() can keep a count of the number of bytes actually
+written, either in space reserved in the sub-buffers themselves or as
+a separate array. See the 'reserve' example in the relay-apps tarball
+at http://relayfs.sourceforge.net for an example of how this can be
+done. Because the write is under control of the client and is
+separated from the reserve, relay_reserve() doesn't protect the buffer
+at all - it's up to the client to provide the appropriate
+synchronization when using relay_reserve().
+
+Closing a channel
+-----------------
+
+The client calls relay_close() when it's finished using the channel.
+The channel and its associated buffers are destroyed when there are no
+longer any references to any of the channel buffers. relay_flush()
+forces a sub-buffer switch on all the channel buffers, and can be used
+to finalize and process the last sub-buffers before the channel is
+closed.
+
+Misc
+----
+
+Some applications may want to keep a channel around and re-use it
+rather than open and close a new channel for each use. relay_reset()
+can be used for this purpose - it resets a channel to its initial
+state without reallocating channel buffer memory or destroying
+existing mappings. It should however only be called when it's safe to
+do so, i.e. when the channel isn't currently being written to.
+
+Finally, there are a couple of utility callbacks that can be used for
+different purposes. buf_mapped() is called whenever a channel buffer
+is mmapped from user space and buf_unmapped() is called when it's
+unmapped. The client can use this notification to trigger actions
+within the kernel application, such as enabling/disabling logging to
+the channel.
+
+
+Resources
+=========
+
+For news, example code, mailing list, etc. see the relay interface homepage:
+
+ http://relayfs.sourceforge.net
+
+
+Credits
+=======
+
+The ideas and specs for the relay interface came about as a result of
+discussions on tracing involving the following:
+
+Michel Dagenais <michel.dagenais@xxxxxxxxxx>
+Richard Moore <richardj_moore@xxxxxxxxxx>
+Bob Wisniewski <bob@xxxxxxxxxxxxxx>
+Karim Yaghmour <karim@xxxxxxxxxxx>
+Tom Zanussi <zanussi@xxxxxxxxxx>
+
+Also thanks to Hubertus Franke for a lot of useful suggestions and bug
+reports.
diff --git a/Documentation/filesystems/relayfs.txt b/Documentation/filesystems/relayfs.txt
deleted file mode 100644
index 5832377..0000000
--- a/Documentation/filesystems/relayfs.txt
+++ /dev/null
@@ -1,442 +0,0 @@
-
-relayfs - a high-speed data relay filesystem
-============================================
-
-relayfs is a filesystem designed to provide an efficient mechanism for
-tools and facilities to relay large and potentially sustained streams
-of data from kernel space to user space.
-
-The main abstraction of relayfs is the 'channel'. A channel consists
-of a set of per-cpu kernel buffers each represented by a file in the
-relayfs filesystem. Kernel clients write into a channel using
-efficient write functions which automatically log to the current cpu's
-channel buffer. User space applications mmap() the per-cpu files and
-retrieve the data as it becomes available.
-
-The format of the data logged into the channel buffers is completely
-up to the relayfs client; relayfs does however provide hooks which
-allow clients to impose some structure on the buffer data. Nor does
-relayfs implement any form of data filtering - this also is left to
-the client. The purpose is to keep relayfs as simple as possible.
-
-This document provides an overview of the relayfs API. The details of
-the function parameters are documented along with the functions in the
-filesystem code - please see that for details.
-
-Semantics
-=========
-
-Each relayfs channel has one buffer per CPU, each buffer has one or
-more sub-buffers. Messages are written to the first sub-buffer until
-it is too full to contain a new message, in which case it it is
-written to the next (if available). Messages are never split across
-sub-buffers. At this point, userspace can be notified so it empties
-the first sub-buffer, while the kernel continues writing to the next.
-
-When notified that a sub-buffer is full, the kernel knows how many
-bytes of it are padding i.e. unused. Userspace can use this knowledge
-to copy only valid data.
-
-After copying it, userspace can notify the kernel that a sub-buffer
-has been consumed.
-
-relayfs can operate in a mode where it will overwrite data not yet
-collected by userspace, and not wait for it to consume it.
-
-relayfs itself does not provide for communication of such data between
-userspace and kernel, allowing the kernel side to remain simple and
-not impose a single interface on userspace. It does provide a set of
-examples and a separate helper though, described below.
-
-klog and relay-apps example code
-================================
-
-relayfs itself is ready to use, but to make things easier, a couple
-simple utility functions and a set of examples are provided.
-
-The relay-apps example tarball, available on the relayfs sourceforge
-site, contains a set of self-contained examples, each consisting of a
-pair of .c files containing boilerplate code for each of the user and
-kernel sides of a relayfs application; combined these two sets of
-boilerplate code provide glue to easily stream data to disk, without
-having to bother with mundane housekeeping chores.
-
-The 'klog debugging functions' patch (klog.patch in the relay-apps
-tarball) provides a couple of high-level logging functions to the
-kernel which allow writing formatted text or raw data to a channel,
-regardless of whether a channel to write into exists or not, or
-whether relayfs is compiled into the kernel or is configured as a
-module. These functions allow you to put unconditional 'trace'
-statements anywhere in the kernel or kernel modules; only when there
-is a 'klog handler' registered will data actually be logged (see the
-klog and kleak examples for details).
-
-It is of course possible to use relayfs from scratch i.e. without
-using any of the relay-apps example code or klog, but you'll have to
-implement communication between userspace and kernel, allowing both to
-convey the state of buffers (full, empty, amount of padding).
-
-klog and the relay-apps examples can be found in the relay-apps
-tarball on http://relayfs.sourceforge.net
-
-
-The relayfs user space API
-==========================
-
-relayfs implements basic file operations for user space access to
-relayfs channel buffer data. Here are the file operations that are
-available and some comments regarding their behavior:
-
-open() enables user to open an _existing_ buffer.
-
-mmap() results in channel buffer being mapped into the caller's
- memory space. Note that you can't do a partial mmap - you must
- map the entire file, which is NRBUF * SUBBUFSIZE.
-
-read() read the contents of a channel buffer. The bytes read are
- 'consumed' by the reader i.e. they won't be available again
- to subsequent reads. If the channel is being used in
- no-overwrite mode (the default), it can be read at any time
- even if there's an active kernel writer. If the channel is
- being used in overwrite mode and there are active channel
- writers, results may be unpredictable - users should make
- sure that all logging to the channel has ended before using
- read() with overwrite mode.
-
-poll() POLLIN/POLLRDNORM/POLLERR supported. User applications are
- notified when sub-buffer boundaries are crossed.
-
-close() decrements the channel buffer's refcount. When the refcount
- reaches 0 i.e. when no process or kernel client has the buffer
- open, the channel buffer is freed.
-
-
-In order for a user application to make use of relayfs files, the
-relayfs filesystem must be mounted. For example,
-
- mount -t relayfs relayfs /mnt/relay
-
-NOTE: relayfs doesn't need to be mounted for kernel clients to create
- or use channels - it only needs to be mounted when user space
- applications need access to the buffer data.
-
-
-The relayfs kernel API
-======================
-
-Here's a summary of the API relayfs provides to in-kernel clients:
-
-
- channel management functions:
-
- relay_open(base_filename, parent, subbuf_size, n_subbufs,
- callbacks)
- relay_close(chan)
- relay_flush(chan)
- relay_reset(chan)
- relayfs_create_dir(name, parent)
- relayfs_remove_dir(dentry)
- relayfs_create_file(name, parent, mode, fops, data)
- relayfs_remove_file(dentry)
-
- channel management typically called on instigation of userspace:
-
- relay_subbufs_consumed(chan, cpu, subbufs_consumed)
-
- write functions:
-
- relay_write(chan, data, length)
- __relay_write(chan, data, length)
- relay_reserve(chan, length)
-
- callbacks:
-
- subbuf_start(buf, subbuf, prev_subbuf, prev_padding)
- buf_mapped(buf, filp)
- buf_unmapped(buf, filp)
- create_buf_file(filename, parent, mode, buf, is_global)
- remove_buf_file(dentry)
-
- helper functions:
-
- relay_buf_full(buf)
- subbuf_start_reserve(buf, length)
-
-
-Creating a channel
-------------------
-
-relay_open() is used to create a channel, along with its per-cpu
-channel buffers. Each channel buffer will have an associated file
-created for it in the relayfs filesystem, which can be opened and
-mmapped from user space if desired. The files are named
-basename0...basenameN-1 where N is the number of online cpus, and by
-default will be created in the root of the filesystem. If you want a
-directory structure to contain your relayfs files, you can create it
-with relayfs_create_dir() and pass the parent directory to
-relay_open(). Clients are responsible for cleaning up any directory
-structure they create when the channel is closed - use
-relayfs_remove_dir() for that.
-
-The total size of each per-cpu buffer is calculated by multiplying the
-number of sub-buffers by the sub-buffer size passed into relay_open().
-The idea behind sub-buffers is that they're basically an extension of
-double-buffering to N buffers, and they also allow applications to
-easily implement random-access-on-buffer-boundary schemes, which can
-be important for some high-volume applications. The number and size
-of sub-buffers is completely dependent on the application and even for
-the same application, different conditions will warrant different
-values for these parameters at different times. Typically, the right
-values to use are best decided after some experimentation; in general,
-though, it's safe to assume that having only 1 sub-buffer is a bad
-idea - you're guaranteed to either overwrite data or lose events
-depending on the channel mode being used.
-
-Channel 'modes'
----------------
-
-relayfs channels can be used in either of two modes - 'overwrite' or
-'no-overwrite'. The mode is entirely determined by the implementation
-of the subbuf_start() callback, as described below. In 'overwrite'
-mode, also known as 'flight recorder' mode, writes continuously cycle
-around the buffer and will never fail, but will unconditionally
-overwrite old data regardless of whether it's actually been consumed.
-In no-overwrite mode, writes will fail i.e. data will be lost, if the
-number of unconsumed sub-buffers equals the total number of
-sub-buffers in the channel. It should be clear that if there is no
-consumer or if the consumer can't consume sub-buffers fast enought,
-data will be lost in either case; the only difference is whether data
-is lost from the beginning or the end of a buffer.
-
-As explained above, a relayfs channel is made of up one or more
-per-cpu channel buffers, each implemented as a circular buffer
-subdivided into one or more sub-buffers. Messages are written into
-the current sub-buffer of the channel's current per-cpu buffer via the
-write functions described below. Whenever a message can't fit into
-the current sub-buffer, because there's no room left for it, the
-client is notified via the subbuf_start() callback that a switch to a
-new sub-buffer is about to occur. The client uses this callback to 1)
-initialize the next sub-buffer if appropriate 2) finalize the previous
-sub-buffer if appropriate and 3) return a boolean value indicating
-whether or not to actually go ahead with the sub-buffer switch.
-
-To implement 'no-overwrite' mode, the userspace client would provide
-an implementation of the subbuf_start() callback something like the
-following:
-
-static int subbuf_start(struct rchan_buf *buf,
- void *subbuf,
- void *prev_subbuf,
- unsigned int prev_padding)
-{
- if (prev_subbuf)
- *((unsigned *)prev_subbuf) = prev_padding;
-
- if (relay_buf_full(buf))
- return 0;
-
- subbuf_start_reserve(buf, sizeof(unsigned int));
-
- return 1;
-}
-
-If the current buffer is full i.e. all sub-buffers remain unconsumed,
-the callback returns 0 to indicate that the buffer switch should not
-occur yet i.e. until the consumer has had a chance to read the current
-set of ready sub-buffers. For the relay_buf_full() function to make
-sense, the consumer is reponsible for notifying relayfs when
-sub-buffers have been consumed via relay_subbufs_consumed(). Any
-subsequent attempts to write into the buffer will again invoke the
-subbuf_start() callback with the same parameters; only when the
-consumer has consumed one or more of the ready sub-buffers will
-relay_buf_full() return 0, in which case the buffer switch can
-continue.
-
-The implementation of the subbuf_start() callback for 'overwrite' mode
-would be very similar:
-
-static int subbuf_start(struct rchan_buf *buf,
- void *subbuf,
- void *prev_subbuf,
- unsigned int prev_padding)
-{
- if (prev_subbuf)
- *((unsigned *)prev_subbuf) = prev_padding;
-
- subbuf_start_reserve(buf, sizeof(unsigned int));
-
- return 1;
-}
-
-In this case, the relay_buf_full() check is meaningless and the
-callback always returns 1, causing the buffer switch to occur
-unconditionally. It's also meaningless for the client to use the
-relay_subbufs_consumed() function in this mode, as it's never
-consulted.
-
-The default subbuf_start() implementation, used if the client doesn't
-define any callbacks, or doesn't define the subbuf_start() callback,
-implements the simplest possible 'no-overwrite' mode i.e. it does
-nothing but return 0.
-
-Header information can be reserved at the beginning of each sub-buffer
-by calling the subbuf_start_reserve() helper function from within the
-subbuf_start() callback. This reserved area can be used to store
-whatever information the client wants. In the example above, room is
-reserved in each sub-buffer to store the padding count for that
-sub-buffer. This is filled in for the previous sub-buffer in the
-subbuf_start() implementation; the padding value for the previous
-sub-buffer is passed into the subbuf_start() callback along with a
-pointer to the previous sub-buffer, since the padding value isn't
-known until a sub-buffer is filled. The subbuf_start() callback is
-also called for the first sub-buffer when the channel is opened, to
-give the client a chance to reserve space in it. In this case the
-previous sub-buffer pointer passed into the callback will be NULL, so
-the client should check the value of the prev_subbuf pointer before
-writing into the previous sub-buffer.
-
-Writing to a channel
---------------------
-
-kernel clients write data into the current cpu's channel buffer using
-relay_write() or __relay_write(). relay_write() is the main logging
-function - it uses local_irqsave() to protect the buffer and should be
-used if you might be logging from interrupt context. If you know
-you'll never be logging from interrupt context, you can use
-__relay_write(), which only disables preemption. These functions
-don't return a value, so you can't determine whether or not they
-failed - the assumption is that you wouldn't want to check a return
-value in the fast logging path anyway, and that they'll always succeed
-unless the buffer is full and no-overwrite mode is being used, in
-which case you can detect a failed write in the subbuf_start()
-callback by calling the relay_buf_full() helper function.
-
-relay_reserve() is used to reserve a slot in a channel buffer which
-can be written to later. This would typically be used in applications
-that need to write directly into a channel buffer without having to
-stage data in a temporary buffer beforehand. Because the actual write
-may not happen immediately after the slot is reserved, applications
-using relay_reserve() can keep a count of the number of bytes actually
-written, either in space reserved in the sub-buffers themselves or as
-a separate array. See the 'reserve' example in the relay-apps tarball
-at http://relayfs.sourceforge.net for an example of how this can be
-done. Because the write is under control of the client and is
-separated from the reserve, relay_reserve() doesn't protect the buffer
-at all - it's up to the client to provide the appropriate
-synchronization when using relay_reserve().
-
-Closing a channel
------------------
-
-The client calls relay_close() when it's finished using the channel.
-The channel and its associated buffers are destroyed when there are no
-longer any references to any of the channel buffers. relay_flush()
-forces a sub-buffer switch on all the channel buffers, and can be used
-to finalize and process the last sub-buffers before the channel is
-closed.
-
-Creating non-relay files
-------------------------
-
-relay_open() automatically creates files in the relayfs filesystem to
-represent the per-cpu kernel buffers; it's often useful for
-applications to be able to create their own files alongside the relay
-files in the relayfs filesystem as well e.g. 'control' files much like
-those created in /proc or debugfs for similar purposes, used to
-communicate control information between the kernel and user sides of a
-relayfs application. For this purpose the relayfs_create_file() and
-relayfs_remove_file() API functions exist. For relayfs_create_file(),
-the caller passes in a set of user-defined file operations to be used
-for the file and an optional void * to a user-specified data item,
-which will be accessible via inode->u.generic_ip (see the relay-apps
-tarball for examples). The file_operations are a required parameter
-to relayfs_create_file() and thus the semantics of these files are
-completely defined by the caller.
-
-See the relay-apps tarball at http://relayfs.sourceforge.net for
-examples of how these non-relay files are meant to be used.
-
-Creating relay files in other filesystems
------------------------------------------
-
-By default of course, relay_open() creates relay files in the relayfs
-filesystem. Because relay_file_operations is exported, however, it's
-also possible to create and use relay files in other pseudo-filesytems
-such as debugfs.
-
-For this purpose, two callback functions are provided,
-create_buf_file() and remove_buf_file(). create_buf_file() is called
-once for each per-cpu buffer from relay_open() to allow the client to
-create a file to be used to represent the corresponding buffer; if
-this callback is not defined, the default implementation will create
-and return a file in the relayfs filesystem to represent the buffer.
-The callback should return the dentry of the file created to represent
-the relay buffer. Note that the parent directory passed to
-relay_open() (and passed along to the callback), if specified, must
-exist in the same filesystem the new relay file is created in. If
-create_buf_file() is defined, remove_buf_file() must also be defined;
-it's responsible for deleting the file(s) created in create_buf_file()
-and is called during relay_close().
-
-The create_buf_file() implementation can also be defined in such a way
-as to allow the creation of a single 'global' buffer instead of the
-default per-cpu set. This can be useful for applications interested
-mainly in seeing the relative ordering of system-wide events without
-the need to bother with saving explicit timestamps for the purpose of
-merging/sorting per-cpu files in a postprocessing step.
-
-To have relay_open() create a global buffer, the create_buf_file()
-implementation should set the value of the is_global outparam to a
-non-zero value in addition to creating the file that will be used to
-represent the single buffer. In the case of a global buffer,
-create_buf_file() and remove_buf_file() will be called only once. The
-normal channel-writing functions e.g. relay_write() can still be used
-- writes from any cpu will transparently end up in the global buffer -
-but since it is a global buffer, callers should make sure they use the
-proper locking for such a buffer, either by wrapping writes in a
-spinlock, or by copying a write function from relayfs_fs.h and
-creating a local version that internally does the proper locking.
-
-See the 'exported-relayfile' examples in the relay-apps tarball for
-examples of creating and using relay files in debugfs.
-
-Misc
-----
-
-Some applications may want to keep a channel around and re-use it
-rather than open and close a new channel for each use. relay_reset()
-can be used for this purpose - it resets a channel to its initial
-state without reallocating channel buffer memory or destroying
-existing mappings. It should however only be called when it's safe to
-do so i.e. when the channel isn't currently being written to.
-
-Finally, there are a couple of utility callbacks that can be used for
-different purposes. buf_mapped() is called whenever a channel buffer
-is mmapped from user space and buf_unmapped() is called when it's
-unmapped. The client can use this notification to trigger actions
-within the kernel application, such as enabling/disabling logging to
-the channel.
-
-
-Resources
-=========
-
-For news, example code, mailing list, etc. see the relayfs homepage:
-
- http://relayfs.sourceforge.net
-
-
-Credits
-=======
-
-The ideas and specs for relayfs came about as a result of discussions
-on tracing involving the following:
-
-Michel Dagenais <michel.dagenais@xxxxxxxxxx>
-Richard Moore <richardj_moore@xxxxxxxxxx>
-Bob Wisniewski <bob@xxxxxxxxxxxxxx>
-Karim Yaghmour <karim@xxxxxxxxxxx>
-Tom Zanussi <zanussi@xxxxxxxxxx>
-
-Also thanks to Hubertus Franke for a lot of useful suggestions and bug
-reports.
-
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