linux-kernel - Re: [tip:sched/core] sched, time: Switch VIRT_CPU_ACCOUNTING

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Message-ID: <CAFTL4hwdWhkeOSCF7Y0Qkuy6DjJ_-N8jJJO68ob=pKtps1jiGA@mail.gmail.com>
Date:	Mon, 29 Feb 2016 16:31:56 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mike Galbraith <efault@....de>, Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched, time: Switch VIRT_CPU_ACCOUNTING_GEN to
 jiffy granularity

2016-02-29 12:18 GMT+01:00 tip-bot for Rik van Riel <tipbot@...or.com>:
> Commit-ID:  ff9a9b4c4334b53b52ee9279f30bd5dd92ea9bdd
> Gitweb:     http://git.kernel.org/tip/ff9a9b4c4334b53b52ee9279f30bd5dd92ea9bdd
> Author:     Rik van Riel <riel@...hat.com>
> AuthorDate: Wed, 10 Feb 2016 20:08:27 -0500
> Committer:  Ingo Molnar <mingo@...nel.org>
> CommitDate: Mon, 29 Feb 2016 09:53:10 +0100
>
> sched, time: Switch VIRT_CPU_ACCOUNTING_GEN to jiffy granularity
>
> When profiling syscall overhead on nohz-full kernels,
> after removing __acct_update_integrals() from the profile,
> native_sched_clock() remains as the top CPU user. This can be
> reduced by moving VIRT_CPU_ACCOUNTING_GEN to jiffy granularity.
>
> This will reduce timing accuracy on nohz_full CPUs to jiffy
> based sampling, just like on normal CPUs. It results in
> totally removing native_sched_clock from the profile, and
> significantly speeding up the syscall entry and exit path,
> as well as irq entry and exit, and KVM guest entry & exit.
>
> Additionally, only call the more expensive functions (and
> advance the seqlock) when jiffies actually changed.
>
> This code relies on another CPU advancing jiffies when the
> system is busy. On a nohz_full system, this is done by a
> housekeeping CPU.
>
> A microbenchmark calling an invalid syscall number 10 million
> times in a row speeds up an additional 30% over the numbers
> with just the previous patches, for a total speedup of about
> 40% over 4.4 and 4.5-rc1.
>
> Run times for the microbenchmark:
>
>  4.4                            3.8 seconds
>  4.5-rc1                        3.7 seconds
>  4.5-rc1 + first patch          3.3 seconds
>  4.5-rc1 + first 3 patches      3.1 seconds
>  4.5-rc1 + all patches          2.3 seconds
>
> A non-NOHZ_FULL cpu (not the housekeeping CPU):
>
>  all kernels                    1.86 seconds
>
> Signed-off-by: Rik van Riel <riel@...hat.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Mike Galbraith <efault@....de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: clark@...hat.com
> Cc: eric.dumazet@...il.com
> Cc: fweisbec@...il.com

It seems the tip bot doesn't parse correctly the Cc tags as I wasn't
cc'ed on this commit.

Also I wish I had a chance to test and ack this patch before it got
applied. I guess I should have told I was in vacation for the last
weeks.

I'm going to run it through some tests.

Thanks.