On Mon, Nov 06, 2017 at 07:12:58PM +0300, Kirill Tkhai wrote: > >> + atomic_inc(&tg->stat[rq->cpu].nr_iowait); > > > > You're joking right, more atomic ops on the fast paths.. > > There should be a synchronization... It's modified under rq->lock everywhere, > except try_to_wakeup(). > Would it be better to use one more rq->lock at try_to_wakeup() instead of > atomic?
No, of course not. We spend a lot of time getting of that rq->lock there. The better option is to not care about iowait, since its a complete garbage number to begin with -- read that commit I pointed you to. But if you do manage to convince me iowait is a sane thing to export (and its not); then you should not use atomics -- nor is there any need to. Since all you want to export is \Sum nr_iowait, you can inc/dec to pure cpu local variables and the sum will make it all work. The extant iowait crap cannot do this because it thinks per-cpu IO-wait is a thing -- its not, its a random number at best, but its ABI so we can't fix :-(
