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

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Date:	Mon, 2 Mar 2015 13:44:04 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jason Low <jason.low2@...com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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 Mon, Mar 2, 2015 at 1:16 PM, Jason Low <jason.low2@...com> wrote:
>
> In original code, we set cputimer->running first so it is running while
> we call update_gt_cputime(). Now in this patch, we swapped the 2 calls
> such that we set running after calling update_gt_cputime(), so that
> wouldn't be an issue anymore.

Hmm. If you actually care about ordering, and 'running' should be
written to after the other things, then it might be best if you use

   smp_store_release(&cputimer->running, 1);

which makes it clear that the store happens *after* what went before it.

Or at least have a "smp_wmb()" between the atomic64 updates and the
"WRITE_ONCE()".

I guess that since you use cmpxchg in update_gt_cputime, the accesses
end up being ordered anyway, but it might be better to make that thing
very explicit.

                   Linus
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