On Wed, 21 Oct 2020, Mel Gorman wrote: > On Wed, Oct 21, 2020 at 02:56:06PM +0200, Julia Lawall wrote: > > > > > > On Wed, 21 Oct 2020, Mel Gorman wrote: > > > > > On Wed, Oct 21, 2020 at 02:25:32PM +0200, Vincent Guittot wrote: > > > > > I see Vincent already agreed with the patch so I could be wrong. Vincent, > > > > > did I miss something stupid? > > > > > > > > This patch fixes the problem that we don't favor anymore the prev_cpu when it is idle since > > > > commit 11f10e5420f6ce because load is not null when cpu is idle whereas runnable_load was > > > > And this is important because this will then decide in which LLC we will looks for a cpu > > > > > > > > > > Ok, that is understandable but I'm still concerned that the fix simply > > > trades one problem for another by leaving related tasks remote to each > > > other and increasing cache misses and remote data accesses. > > > > > > wake_affine_weight is a giant pain because really we don't care about the > > > load on the waker CPU or its available, we care about whether it has idle > > > siblings that can be found quickly. As tempting as ripping it out is, > > > it never happened because sometimes it makes the right decision. > > > > My goal was to restore the previous behavior, when runnable load was used. > > The patch removing the use of runnable load (11f10e5420f6) presented it > > basically as that load balancing was using it, so wakeup should use it > > too, and any way it didn't matter because idle CPUS were checked for > > anyway. > > > > Which is fair. > > > Is your point of view that the proposed change is overkill? Or is it that > > the original behavior was not desirable? > > > > I worry it's overkill because prev is always used if it is idle even > if it is on a node remote to the waker. It cuts off the option of a > wakee moving to a CPU local to the waker which is not equivalent to the > original behaviour. Could it be possible to check p->recent_used_cpu? If that is prev (or on the same socket?), then prev could be a good choice. If that is on the same socket as the waker, then maybe the waker would be better. julia
