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Message-ID: <1425336200.5304.84.camel@j-VirtualBox>
Subject: Re: [PATCH v2] sched, timer: Use atomics for thread_group_cputimer
 to improve scalability
From: Jason Low <jason.low2@hp.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Frederic Weisbecker <fweisbec@gmail.com>,
        Rik van Riel <riel@redhat.com>, Steven Rostedt <rostedt@goodmis.org>,
        Scott Norton <scott.norton@hp.com>,
        Aswin Chandramouleeswaran <aswin@hp.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        jason.low2@hp.com
Date: Mon, 02 Mar 2015 14:43:20 -0800
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On Mon, 2015-03-02 at 13:44 -0800, Linus Torvalds wrote:
> On Mon, Mar 2, 2015 at 1:16 PM, Jason Low <jason.low2@hp.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.

Yeah, I suppose the extra (smp_mb or smp_wmb) might add more overhead
but since this is not a common code path anyway, it would be worth
adding it to make things more clear.