linux-kernel - Re: [PATCH 1/9] perf_counter: unify and fix delayed counter wakeup

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Date:	Sun, 29 Mar 2009 11:16:41 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Eric Dumazet <dada1@...mosbay.com>
Subject: Re: [PATCH 1/9] perf_counter: unify and fix delayed counter wakeup

On Sun, 2009-03-29 at 11:14 +1100, Paul Mackerras wrote:
> Peter Zijlstra writes:
> 
> > While going over the wakeup code I noticed delayed wakeups only work
> > for hardware counters but basically all software counters rely on
> > them.
> 
> Hmmm, I don't like the extra latency this introduces, particularly
> since on powerpc we already have a good way to avoid the latency.

Right, so I can re-instate the powerpc bits and have it call
perf_counter_do_pending() whenever it finds the per-cpu pending bit set.

I'd have to look into the fancy new per-cpu stuff for the x86 bits, but
I'm reasonably sure something like that should be doable.

> I did a grep for perf_swcounter_event calls that have nmi=1, and there
> are a couple, to my surprise.  Why does the context switch one have
> nmi=1?  It certainly isn't called from an actual NMI handler.  Is it
> because of locking issues?

Yeah, can't do a wakeup while holding the rq->lock.

> The other one is the tracepoint call in perf_tpcounter_event.  I
> assume you have put nmi=1 there because you don't know what context
> we're in.  That means we'll always delay the wakeup even when we might
> be in an ordinary interrupt-on process context.  Couldn't we do
> better?

Maybe, not only real in_nmi() tracepoints have that problem, we also
have lock_acquire() like tracepoints that could call into the event code
in the middle of a lock acquisition (which might be rq->lock).

So always using nmi=1 for those seemed like the safe way out.

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