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

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Message-ID: <CA+55aFwyC5YDKgmc_FFi7QEDO_5io+2y-xneMtyuHApsURGaxg@mail.gmail.com>
Date:	Thu, 19 Mar 2015 10:59:08 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jason Low <jason.low2@...com>
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>
Subject: Re: [PATCH v2] sched, timer: Use atomics for thread_group_cputimer to
 improve scalability

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,

                      Linus
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