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

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Date:	Sun, 29 Mar 2009 11:14:43 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
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

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.

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?

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?

Paul.
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