linux-kernel - Re: perfmon3 interface overview

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Date:	Sat, 4 Oct 2008 16:05:06 +1000
From:	David Gibson <david@...son.dropbear.id.au>
To:	eranian@...il.com
Cc:	linux-kernel@...r.kernel.org,
	perfmon2-devel <perfmon2-devel@...ts.sourceforge.net>
Subject: 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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