linux-kernel - Re: [PATCH RFC] time: drop do_sys

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Date:	Wed, 13 Aug 2014 09:24:06 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>
CC:	Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Frank Mayhar <fmayhar@...gle.com>,
	Frederic Weisbecker <fweisbec@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sanjay Rao <srao@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>
Subject: Re: [PATCH RFC] time: drop do_sys_times spinlock

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On 08/13/2014 07:11 AM, Peter Zijlstra wrote:
> On Wed, Aug 13, 2014 at 08:59:50AM +0200, Mike Galbraith wrote:
> 
>> I was told that clock_gettime(CLOCK_PROCESS_CPUTIME_ID) has
>> scalability issues on BIG boxen
> 
>> I'm sure the real clock_gettime() using proggy that gummed up a
>> ~1200 core box for "a while" wasn't the testcase below, which
>> will gum it up for a long while, but looks to me like using
>> CLOCK_PROCESS_CPUTIME_ID from LOTS of threads is a "Don't do
>> that, it'll hurt a LOT".
> 
> Yes, don't do that. Its unavoidably slow and bad.

I don't see why that needs the tasklist_lock, when do_sys_times
grabs a different lock.

If the same bottleneck exists from multiple places, maybe it does
make sense to have a seqlock for the statistics at the sighand
level?

I can code up a patch that does that, and throw it over the wall
to people with big systems who hit that bottleneck on a regular
basis...

- -- 
All rights reversed
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