Re: perfmon3 interface overview

On Fri, Oct 03, 2008 at 01:12:09PM +0200, stephane eranian wrote:
David,
[snip]
III) attaching and detaching

With v2.81:
int pfm_load_context(int fd, pfarg_load_t *load);
int pfm_unload_context(int fd);

With v3.0:
int pfm_attach_session(int fd, int flags, int target);
int pfm_detach_session(int fd, int flags);

Couldn't you get rid of one more syscall here by making detach a
special case of attach with a special "null" value for target, or a
special flag?


We could combine the two and use the flag field to indicate attach/detach.
The target is not a pointer but an int. Some people suggested I use an
unsigned long instead. In anycase, we could not use 0 to indicate "detach"
because CPU0 is a valid choice for system-wide. Thus we would have to
pick another value to mean "nothing", e.g, -1.

> IV) starting and stopping
>
> With v2.81:
> int pfm_start(int fd, pfarg_start_t *st);
> int pfm_stop(int fd);
> int pfm_restart(int fd);
>
> With v3.0:
> int pfm_start_session(int fd, int flags);
> int pfm_stop_session(int fd, int flags);

Likewise, couldn't you cut this down by one more syscall by making it
int pfm_set_session_state(int fd, int flags);
and having a 'RUNNING' flag, which selects start or stop behaviour?

That one we can certainly do. That's a good idea.

Some more thoughts on this.

If we wanted to go even further, we could combine start/stop, attach/detach
into a single syscall:

Well, you could. But the attach/detach take a parameter which
start/stop don't, making it a less obvious merge to make.

int pfm_control_session(int fd, int flags, int target);
With flags:
PFM_CTFL_START : start monitoring
PFM_CTFL_STOP : stop monitoring
PFM_CTFL_RESTART: resume after overflow notification

PFM_CTFL_ATTACH: attach to thread or cpu designated by 'target'
PFM_CTFL_DETACH: detach session

But then, this is a form of ioctl() which people don't like....

--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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