linux-kernel - Re: [PATCH] perf: reduce stack usage of schedule

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Message-ID: <20130531155629.GH27176@twins.programming.kicks-ass.net>
Date:	Fri, 31 May 2013 17:56:29 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrew Hunter <ahh@...gle.com>
Cc:	linux-kernel@...r.kernel.org, eranian@...gle.com, mingo@...hat.com
Subject: Re: [PATCH] perf: reduce stack usage of schedule_events

On Thu, May 23, 2013 at 11:07:03AM -0700, Andrew Hunter wrote:
> schedule_events caches event constraints on the stack during
> scheduling.  Given the number of possible events, this is 512 bytes of
> stack; since it can be invoked under schedule() under god-knows-what,
> this is causing stack blowouts.
> 
> Trade some space usage for stack safety: add a place to cache the
> constraint pointer to struct perf_event.  For 8 bytes per event (1% of
> its size) we can save the giant stack frame.
> 
> This shouldn't change any aspect of scheduling whatsoever and while in
> theory the locality's a tiny bit worse, I doubt we'll see any
> performance impact either.
> 
> Tested: `perf stat whatever` does not blow up and produces
> results that aren't hugely obviously wrong.  I'm not sure how to run
> particularly good tests of perf code, but this should not produce any
> functional change whatsoever.
> 
> Signed-off-by: Andrew Hunter <ahh@...gle.com>
> Reviewed-by: Stephane Eranian <eranian@...gle.com>

OK nothing really strange popped out during a quick read.

Thanks!
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