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

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Date:	Thu, 5 Mar 2015 16:20:34 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jason Low <jason.low2@...com>, 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>,
	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 02, 2015 at 01:44:04PM -0800, Linus Torvalds wrote:
> 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()".

FWIW, perhaps it can be reduced with an smp_mb__before_atomic() on the
account_group_*_time() side, paired with smp_wmb() from the thread_group_cputimer()
side. Arming cputime->running shouldn't be too frequent while update cputime
happens at least every tick...

Assuming smp_mb__before_atomic() is more lightweight than smp_load_acquire()
of course. 

> 
> 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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