linux-kernel - Re: [PATCH 1/1] sched: Make schedstats a runtime tunable that is disabled by default v4

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Message-ID: <20160203112849.GB27850@gmail.com>
Date:	Wed, 3 Feb 2016 12:28:49 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Mel Gorman <mgorman@...hsingularity.net>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Matt Fleming <matt@...eblueprint.co.uk>,
	Mike Galbraith <mgalbraith@...e.de>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] sched: Make schedstats a runtime tunable that is
 disabled by default v4


* Mel Gorman <mgorman@...hsingularity.net> wrote:

> Changelog since v3
> o Force enable stats during profiling and latencytop
> 
> Changelog since V2
> o Print stats that are not related to schedstat
> o Reintroduce a static inline for update_stats_dequeue
> 
> Changelog since V1
> o Introduce schedstat_enabled and address Ingo's feedback
> o More schedstat-only paths eliminated, particularly ttwu_stat
> 
> schedstats is very useful during debugging and performance tuning but it
> incurs overhead. As such, even though it can be disabled at build time,
> it is often enabled as the information is useful.  This patch adds a
> kernel command-line and sysctl tunable to enable or disable schedstats on
> demand. It is disabled by default as someone who knows they need it can
> also learn to enable it when necessary.
> 
> The benefits are workload-dependent but when it gets down to it, the
> difference will be whether cache misses are incurred updating the shared
> stats or not. [...]

Hm, which shared stats are those? I think we should really fix those as well: 
those shared stats should be percpu collected as well, with no extra cache misses 
in any scheduler fast path.

Thanks,

	Ingo