linux-kernel - Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry

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Date:	Fri, 1 May 2015 08:40:44 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	riel@...hat.com
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org, williams@...hat.com,
	luto@...nel.org, bonzini@...hat.com, fweisbec@...hat.com,
	peterz@...radead.org, heiko.carstens@...ibm.com,
	tglx@...utronix.de, Ingo Molnar <mingo@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable
 & enable from context tracking on syscall entry


* riel@...hat.com <riel@...hat.com> wrote:

> From: Rik van Riel <riel@...hat.com>
> 
> On syscall entry with nohz_full on, we enable interrupts, call user_exit,
> disable interrupts, do something, re-enable interrupts, and go on our
> merry way.
> 
> Profiling shows that a large amount of the nohz_full overhead comes
> from the extraneous disabling and re-enabling of interrupts. Andy
> suggested simply not enabling interrupts until after the context
> tracking code has done its thing, which allows us to skip a whole
> interrupt disable & re-enable cycle.
> 
> This patch builds on top of these patches by Paolo:
> https://lkml.org/lkml/2015/4/28/188
> https://lkml.org/lkml/2015/4/29/139
> 
> Together with this patch I posted earlier this week, the syscall path
> on a nohz_full cpu seems to be about 10% faster.
> https://lkml.org/lkml/2015/4/24/394
> 
> My test is a simple microbenchmark that calls getpriority() in a loop
> 10 million times:
> 
> 		run time	system time
> vanilla		5.49s		2.08s
> __acct patch	5.21s		1.92s
> both patches	4.88s		1.71s

Just curious, what are the numbers if you don't have context tracking 
enabled, i.e. without nohz_full?

I.e. what's the baseline we are talking about?

Thanks,

	Ingo
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