linux-kernel - Re: [PATCH 1/2] x86/dumpstack: Optimize save_stack

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Message-ID: <20160708150749.GI30200@lerouge>
Date:	Fri, 8 Jul 2016 17:07:51 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Byungchul Park <byungchul.park@....com>, peterz@...radead.org,
	linux-kernel@...r.kernel.org, walken@...gle.com,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 1/2] x86/dumpstack: Optimize save_stack_trace

On Fri, Jul 08, 2016 at 12:08:19PM +0200, Ingo Molnar wrote:
> 
> * Byungchul Park <byungchul.park@....com> wrote:
> 
> > On Mon, Jul 04, 2016 at 07:27:54PM +0900, Byungchul Park wrote:
> > > I suggested this patch on https://lkml.org/lkml/2016/6/20/22. However,
> > > I want to proceed saperately since it's somewhat independent from each
> > > other. Frankly speaking, I want this patchset to be accepted at first so
> > > that the crossfeature can use this optimized save_stack_trace_norm()
> > > which makes crossrelease work smoothly.
> > 
> > What do you think about this way to improve it?
> 
> I like both of your improvements, the speed up is impressive:
> 
>   [    2.327597] save_stack_trace() takes 87114 ns
>   ...
>   [    2.781694] save_stack_trace() takes 20044 ns
>   ...
>   [    3.103264] save_stack_trace takes 3821 (sched_lock)
> 
> Could you please also measure call graph recording (perf record -g), how much 
> faster does it get with your patches and what are our remaining performance hot 
> spots?

I don't think it will impact much perf because print_context_stack_bp() checks
ops->address() which return perf_callchain_store() which tells if we crossed
the buffer limit.

But it's worth checking anyway.

Thanks.