linux-kernel - Re: [PACTH v9] stacktrace: Eliminate task stack trace duplication

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Message-ID: <20160817065054.GO30192@twins.programming.kicks-ass.net>
Date:	Wed, 17 Aug 2016 08:50:54 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	robert.foss@...labora.com
Cc:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	x86@...nel.org, davem@...emloft.net, yinghan@...gle.com,
	ast@...com, linux@...musvillemoes.dk, borntraeger@...ibm.com,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Rik van Riel <riel@...hat.com>,
	Andrew Bresticker <abrestic@...omium.org>,
	Sameer Nanda <snanda@...omium.org>,
	Filipe Brandenburger <filbranden@...omium.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PACTH v9] stacktrace: Eliminate task stack trace duplication

On Tue, Aug 16, 2016 at 07:12:36PM -0400, robert.foss@...labora.com wrote:
> From: Ying Han <yinghan@...gle.com>
> 
> The problem with small dmesg ring buffer like 512k is that only limited number
> of task traces will be logged. Sometimes we lose important information only
> because of too many duplicated stack traces. This problem occurs when dumping
> lots of stacks in a single operation, such as sysrq-T.
> 
> This patch tries to reduce the duplication of task stack trace in the dump
> message by hashing the task stack. The hashtable is a 32k pre-allocated buffer
> during bootup. Each time if we find the identical task trace in the task stack,
> we dump only the pid of the task which has the task trace dumped. So it is easy
> to back track to the full stack with the pid.
> 
> When we do the hashing, we eliminate garbage entries from stack traces. Those
> entries are still being printed in the dump to provide more debugging
> informations.
> 
> [   53.510162] kworker/0:0     S ffffffff8161d820     0     4      2 0x00000000
> [   53.517237]  ffff88027547de60 0000000000000046 ffffffff812ab840 0000000000000000
> [   53.524663]  ffff880275460080 ffff88027547dfd8 ffff88027547dfd8 ffff88027547dfd8
> [   53.532092]  ffffffff81813020 ffff880275460080 0000000000000000 ffff8808758670c0
> [   53.539521] Call Trace:
> [   53.541974]  [<ffffffff812ab840>] ? cfq_init_queue+0x350/0x350
> [   53.547791]  [<ffffffff81524d49>] schedule+0x29/0x70
> [   53.552761]  [<ffffffff810945a3>] worker_thread+0x233/0x380
> [   53.558318]  [<ffffffff81094370>] ? manage_workers.isra.28+0x230/0x230
> [   53.564839]  [<ffffffff81099a73>] kthread+0x93/0xa0
> [   53.569714]  [<ffffffff8152e6d4>] kernel_thread_helper+0x4/0x10
> [   53.575628]  [<ffffffff810999e0>] ? kthread_worker_fn+0x140/0x140
> [   53.581714]  [<ffffffff8152e6d0>] ? gs_change+0xb/0xb
> [   53.586762] kworker/u:0     S ffffffff8161d820     0     5      2 0x00000000
> [   53.593858]  ffff88027547fe60 0000000000000046 ffffffffa005cc70 0000000000000000
> [   53.601307]  ffff8802754627d0 ffff88027547ffd8 ffff88027547ffd8 ffff88027547ffd8
> [   53.608788]  ffffffff81813020 ffff8802754627d0 0000000000011fc0 ffff8804758670c0
> [   53.616232] Call Trace:
> [   53.618676] <Same stack as pid 4>
> 

You might want to wait a bit and have a look at this:

  https://lkml.kernel.org/r/cover.1471011425.git.jpoimboe@redhat.com