Linus, I was greatly confused and tried to confuse you. Somehow I misunderstood your last version and didn't bother to read it again until now. Sorry for noise and thanks for your explanations. Oleg. On 07/25, Linus Torvalds wrote: > > On Sat, Jul 25, 2020 at 12:28 PM Oleg Nesterov <oleg@xxxxxxxxxx> wrote: > > > > What I tried to say. AFAICS before that commit we had (almost) the same > > behaviour you propose now: unlock_page/etc wakes all the non-exclusive > > waiters up. > > > > No? > > Yes, but no. > > We'd wake them _up_ fairly aggressively, but then they'd be caught on > the bit being set again by the exclusive locker (that we also woke > up). > > So they'd get woken up, and then go to sleep again. > > So the new behavior wakes things up more aggressively (but a different > way), but not by letting them go out of order and early, but simply by > not going back to sleep again. > > So the "wake up more" is very different - now it's about not going to > sleep again, rather than by ordering the wakeup queue. > > We _could_ order the wakeup queue too, and put all non-exclusive > weiters at the head again. And make it *really* aggressive. > > But since one of ourissues has been "latency of walking the wait > queue", I'm not sure we want that. interspesing any blocking waiters - > and stopping the waitqueue walking as a result - might be better under > load. > > Wild handwaving. We could try it, but IO think that really would be a > separate "try this out" patch. > > Right now, I think my patch will likely make for _better_ latencies > for everything. > > Lower latency of non-exclusive waiters (because not going back to > sleep), but also lower latency of walking the wait queue (because > fewer entries, hopefully, and also less contention due to the "not > going back to sleep" noise) > > Linus >
