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

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Message-ID: <CANrsvRMvk_SdaqN-8cr4VJNd=CCrGv__36KwcpdjTUczm=Y0nA@mail.gmail.com>
Date:   Tue, 13 Sep 2016 23:54:17 +0900
From:   Byungchul Park <max.byungchul.park@...il.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Byungchul Park <byungchul.park@....com>, peterz@...radead.org,
        Ingo Molnar <mingo@...nel.org>, tglx@...utronix.de,
        walken@...gle.com, boqun.feng@...il.com, kirill@...temov.name,
        "linux-kernel@...r.kernel.org" <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 10:18 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 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.

Hello,

You are right.

I also think this will probably be obsoleted with yours.
So I didn't modify any details of the patch.
I will take your comment into account if it becomes necessary.

Anyway, crossrelease needs this patch to work smoothly.
That's only reason why I included this patch in the thread.

Thank you,
Byungchul

> 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



-- 
Thanks,
Byungchul