Re: [PATCH v2] sched, timer: Use atomics for thread_group_cputimer to improve scalability

Linus Torvalds
2015-03-02T21:44:08Z

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.

Linus