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

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Date:	Fri, 8 Jul 2016 09:29:29 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Byungchul Park <byungchul.park@....com>, peterz@...radead.org,
	linux-kernel@...r.kernel.org, walken@...gle.com,
	Frédéric Weisbecker <fweisbec@...il.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?
> 
> 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.

Hm, but is print_context_stack_bp() variant really less reliable?  From
what I can tell, its only differences vs print_context_stack() are:

- It doesn't scan the stack for "guesses" (which are 'unreliable' and
  are ignored by the ops->address() callback anyway).

- It stops if ops->address() returns an error (which in this case means
  the array is full anyway).

- It stops if the address isn't a kernel text address.  I think this
  shouldn't normally be possible unless there's some generated code like
  bpf on the stack.  Maybe it could be slightly improved for this case.

So instead of adding a new save_stack_trace_fast() variant, why don't we
just modify the existing save_stack_trace() to use
print_context_stack_bp()?

-- 
Josh