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

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Date:	Fri, 8 Jul 2016 12:08:19 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Byungchul Park <byungchul.park@....com>
Cc:	peterz@...radead.org, linux-kernel@...r.kernel.org,
	walken@...gle.com,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 1/2] x86/dumpstack: Optimize save_stack_trace

* 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?

Could you please merge your patches to the latest -tip tree, because this commit I 
merged earlier today:

  81c2949f7fdc x86/dumpstack: Add show_stack_regs() and use it

conflicts with your patches. (I'll push this commit out later today.)

Also, could you please rename the _norm names to _fast or so, to signal that this 
is a faster but less reliable method to get a stack dump? Nobody knows what 
'_norm' means, but '_fast' is pretty self-explanatory.

Thanks,

	Ingo