linux-kernel - Re: [PATCH v4 0/5] getcpu

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Date:	Wed, 24 Feb 2016 12:07:28 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	linux-api <linux-api@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>, Andrew Hunter <ahh@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Watson <davejwatson@...com>, Chris Lameter <cl@...ux.com>,
	Ben Maurer <bmaurer@...com>, rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH v4 0/5] getcpu_cache system call for 4.6

On February 23, 2016 8:09:23 PM PST, Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
>----- On Feb 23, 2016, at 8:36 PM, H. Peter Anvin hpa@...or.com wrote:
>
>> On 02/23/2016 03:28 PM, Mathieu Desnoyers wrote:
>>> Hi,
>>> 
>>> Here is a patchset implementing a cache for the CPU number of the
>>> currently running thread in user-space.
>>> 
>>> Benchmarks comparing this approach to a getcpu based on system call
>on
>>> ARM show a 44x speedup. They show a 14x speedup on x86-64 compared
>to
>>> executing lsl from a vDSO through glibc.
>>> 
>>> I'm added a man page in the changelog of patch 1/3, which shows an
>>> example usage of this new system call.
>>> 
>>> This series is based on v4.5-rc5, submitted for Linux 4.6.
>>> 
>>> Feedback is welcome,
>>> 
>> 
>> What is the resulting context switch overhead?
>
>The getcpu_cache only adds code to the thread migration path,
>and to the resume notifier. The context switch path per se is
>untouched. I would therefore expect the overhead on context
>switch to be within the noise, except if stuff like hackbench
>would be so sensitive to the size of struct task_struct that
>a single extra pointer added at the end of struct task_struct
>would throw off the benchmarks.
>
>Is that what you are concerned about ?
>
>Thanks,
>
>Mathieu

Yes, I'd like to see numbers.  It is way easy to handwave small changes away, but they add up over time.  Without numbers it is a bit hard to quantify the pro vs con.
-- 
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