linux-kernel - Re: [PATCH v2] sched, timer: Use atomics for thread_group

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Date:	Thu, 19 Mar 2015 13:14:14 -0700
From:	Jason Low <jason.low2@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Scott Norton <scott.norton@...com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	jason.low2@...com
Subject: Re: [PATCH v2] sched, timer: Use atomics for thread_group_cputimer
 to improve scalability

On Thu, 2015-03-19 at 10:59 -0700, Linus Torvalds wrote:
> On Thu, Mar 19, 2015 at 10:21 AM, Jason Low <jason.low2@...com> wrote:
> >
> > I tested this patch on a 32 bit ARM system with 4 cores. Using the
> > generic 64 bit atomics, I did not see any performance change with this
> > patch, and the relevant functions (account_group_*_time(), ect...) don't
> > show up in perf reports.
> 
> Ok.
> 
> > One factor might be because locking/cacheline contention isn't as
> > apparent on smaller systems to begin with, and lib/atomic64.c also
> > mentions that "this is expected to used on systems with small numbers of
> > CPUs (<= 4 or so)".
> 
> Yeah, that's probably a valid argument anyway - 32-bit systems aren't
> really going to be multi-node big systems any more.
> 
> So I'm ok with the patch,

Okay, I will be sending out a v3 patch which will also mention a bit
about its effect on 32 bit systems in the changelog, in addition to the
changes that were discussed about in this thread (using WRITE_ONCE(),
ect...).

Thanks,
Jason

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