Messages in this thread | | | Subject | Re: [PATCH v2] sched, timer: Use atomics for thread_group_cputimer to improve scalability | From | Jason Low <> | 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.
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