Re: [PATCH] perf_counter: allow and require one-page mmap oncounting counters

[Paul Mackerras]

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?

I'd be OK with saying that you can't use stop-and-signal with NMI
counters.  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.

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.

Paul.
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