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

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Date:	Sun, 29 Mar 2009 11:25:10 +0200
From:	Peter Zijlstra <peterz@...radead.org>
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 11:16 +0200, Peter Zijlstra wrote:
> 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.

In a perfect world, I'd introduce a self-ipi on UP and use that. Also,
in that same perfect world, all arches would support cmpxchg()/xchg() so
we could put the whole thing in generic code.
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