linux-kernel - Re: [RFC PATCH 1/2] thread_local_abi system call: caching current CPU number (x86)

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Date:	Sun, 13 Dec 2015 09:04:45 +0100
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	lkml <linux-kernel@...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>,
	Ingo Molnar <mingo@...hat.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>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-api <linux-api@...r.kernel.org>
Subject: Re: [RFC PATCH 1/2] thread_local_abi system call: caching current CPU
 number (x86)

Hello Mathieu

On 12 December 2015 at 13:40, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
> ----- On Dec 11, 2015, at 1:56 PM, Michael Kerrisk mtk.manpages@...il.com wrote:
>
>> Hi Mathieu,
>>
>> On 12/10/2015 04:39 PM, Mathieu Desnoyers wrote:
>>> Expose a new system call allowing threads to register a userspace memory
>>> area where to store the current CPU number. Scheduler migration sets the
>>> TIF_NOTIFY_RESUME flag on the current thread. Upon return to user-space,
>>> a notify-resume handler updates the current CPU value within that
>>> user-space memory area.
>>>
>>> This getcpu cache is an alternative to the sched_getcpu() vdso which has
>>> a few benefits:
>>> - It is faster to do a memory read that to call a vDSO,
>>> - This cached value can be read from within an inline assembly, which
>>>   makes it a useful building block for restartable sequences.
>>>
>>> This approach is inspired by Paul Turner and Andrew Hunter's work
>>> on percpu atomics, which lets the kernel handle restart of critical
>>> sections:
>>> Ref.:
>>> * https://lkml.org/lkml/2015/6/24/665
>>> * https://lwn.net/Articles/650333/
>>> *
>>> http://www.linuxplumbersconf.org/2013/ocw/system/presentations/1695/original/LPC%20-%20PerCpu%20Atomics.pdf
>>>
>>> Benchmarking sched_getcpu() vs tls cache approach. Getting the
>>> current CPU number:
>>
>> Is there a man page for this system call?
>
> Hi Michael,
>
> Not yet. I first want check whether the overall technical
> approach is deemed acceptable before adding documentation.
>
> Adding a manpage is going to be one of the first steps after
> we agree on the syscall interface. Or perhaps you are suggesting
> that adding a manpage at this RFC stage could help the
> interface discussion ?

Yup, that's exactly what I'm suggesting.

Thanks,

Michael
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