Re: How to write a kernel patch to expose a kernel-level function?

Peter T. Breuer wrote:
> John Hasler <john@xxxxxxxxxx> wrote:
>> hzmonte writes:
>>> What I want, and what smp_processor_id() returns, is which logical
>>> processor the calling thread is running on. Is CPU0, CPU1, or CPU2, for
>>> example?
>
>> Why? Of what possible use could that information be? What problem are you
>> trying to solve?
>
> Dunno. Probably some mistaken idea. But anyway it would be simple to do
> - some ioctl which returns what he wants. But doesn't he have the info
> in proc anyway?
>
How would that do any good? When the process makes the kernel call on ioctl,
a process switch will occur and what would the ioctl return? The
processor-id that the process _used to_ run on, not the one it will run on
once the ioctl returns.

He is doomed. The only way I can think of that will work is to examine the
code of the kernel in the area where process switching occurs. I looked in
there a while ago, and it really tries to return a process to the same
processor it ran on before to try to maintain a high cache hit ratio, but
with the puny caches on my processors (1 Megabit L3 caches) compared to
process sizes (50Megabytes and up for some of the large ones), this may be
of only minor importance.

It depends on the working set size of all the processes currently running. I
have a dbms that runs many processes, and does a lot of IO. Runs pretty well
unless I let BOINC run 4 compute-limited processes at nice 19. Running BOINC
this way should take up all the otherwise unused CPU cycles and not
interfere with the higher priority processes, but it spoils the database
application(s). They try to do a disk write and suspend. Then a BOINC
process runs and dirtys up the caches, so when the IO for the dbms completes
and the dbms process resumes, the cache is completely dirty so it runs at
RAM speeds (533MHz) instead of processor speeds (3.06GHz).

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Relevant Pages

  • Re: How to write a kernel patch to expose a kernel-level function?
    ... When the process makes the kernel call on ioctl, ... > with the puny caches on my processors compared to ... > have a dbms that runs many processes, and does a lot of IO. ... > unless I let BOINC run 4 compute-limited processes at nice 19. ...
    (comp.os.linux.misc)
  • Re: How to write a kernel patch to expose a kernel-level function?
    ... But anyway it would be simple to do>> - some ioctl which returns what he wants. ... There is no special extra process switch on ioctl - he'll get whatever cpu is running his ioctl. ... Yes, it is another question if the kernel decided to switch him to a different cpu on the i/o, but his chances of that are low, or at least about as low as with anything he does if there is any affinity at all. ... I looked in> there a while ago, and it really tries to return a process to the same> processor it ran on before to try to maintain a high cache hit ratio, but> with the puny caches on my processors compared to> process sizes, this may be> of only minor importance. ...
    (comp.os.linux.misc)