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Message-Id: <1238573622.8530.2582.camel@twins>
Date: Wed, 01 Apr 2009 10:13:42 +0200
From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
To: Paul Mackerras <paulus@...ba.org>
Cc: Ingo Molnar <mingo@...e.hu>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf_counter: allow and require one-page mmap on
counting counters
On Wed, 2009-04-01 at 13:32 +1100, Paul Mackerras wrote:
> Peter Zijlstra writes:
>
> > On Wed, 2009-03-25 at 20:42 +1100, Paul Mackerras wrote:
> > >
> > > And here's something else that is semi-related: the PAPI guys want a
> > > kind of counter that counts until it overflows, and then sends a
> > > signal to the process and disables itself (and the whole group it's
> > > in).
> >
> > I tried doing this this evening, and its remarkably hard. Disabling a
> > counter relies on reading the time, and taking ctx->lock and such.
> > Things that are impossible to do in NMI context.
>
> So, if I have a group where the leader is a hardware counter set to
> use NMIs, and there is a task_clock software counter in the group,
> don't we hit exactly the same issue with reading the time?
Hmm, I think you're right there. Nasty.
> I'd be OK with saying that you can't use stop-and-signal with NMI
> counters.
Right, so let them use regular IRQs, yes, that will work. Let me code
that.
> There will still be some issues on powerpc because of our
> lazy interrupt disabling scheme, so some work might have to get
> deferred until we soft-enable interrupts, but we have a way to manage
> that.
Hmm, you're saying ppc always uses NMIs, even when !hw_event.nmi?
> On another topic, I noticed that we have a race with perf_counter_read
> where we do the IPI but don't check in __read() on the destination cpu
> that the task we're after is still running on that cpu. It needs
> checking and retry logic like we have in other places in
> perf_counter.c.
Yep, you're right again. Should we perhaps generalize that whole code
and provide a method vector for the various bits. That way we could
collapse all that code replication.
This TODO list keeps growing :-)
BTW, how's progress with the lazy switching?
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