linux-kernel - Re: [PATCH v3 01/15] x86/dumpstack: Optimize save_stack

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Message-ID: <20160913131802.oiwxgpmccn7uufef@treble>
Date:   Tue, 13 Sep 2016 08:18:02 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Byungchul Park <byungchul.park@....com>
Cc:     peterz@...radead.org, mingo@...nel.org, tglx@...utronix.de,
        walken@...gle.com, boqun.feng@...il.com, kirill@...temov.name,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
        npiggin@...il.com
Subject: Re: [PATCH v3 01/15] x86/dumpstack: Optimize save_stack_trace

On Tue, Sep 13, 2016 at 06:45:00PM +0900, Byungchul Park wrote:
> Currently, x86 implementation of save_stack_trace() is walking all stack
> region word by word regardless of what the trace->max_entries is.
> However, it's unnecessary to walk after already fulfilling caller's
> requirement, say, if trace->nr_entries >= trace->max_entries is true.
> 
> I measured its overhead and printed its difference of sched_clock() with
> my QEMU x86 machine. The latency was improved over 70% when
> trace->max_entries = 5.

This code will (probably) be obsoleted soon with my new unwinder.

Also, my previous comment was ignored:

  Instead of adding a new callback, why not just check the ops->address()
  return value?  It already returns an error if the array is full. 
   
  I think that would be cleaner and would help prevent more callback
  sprawl.

-- 
Josh