On Wed, 21 Oct 2020, Vincent Guittot wrote: > On Wed, 21 Oct 2020 at 17:18, Julia Lawall <julia.lawall@xxxxxxxx> wrote: > > > > > > > > On Wed, 21 Oct 2020, Mel Gorman wrote: > > > > > On Wed, Oct 21, 2020 at 03:24:48PM +0200, Julia Lawall wrote: > > > > > 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. > > > > > > > > But it is equal to the original behavior in the idle prev case if you go > > > > back to the runnable load average days... > > > > > > > > > > It is similar but it misses the sync treatment and sd->imbalance_pct part of > > > wake_affine_weight which has unpredictable consequences. The data > > > available is only on the fully utilised case. > > > > OK, what if my patch were: > > > > @@ -5800,6 +5800,9 @@ wake_affine_idle(int this_cpu, int prev_cpu, int sync) > > if (sync && cpu_rq(this_cpu)->nr_running == 1) > > return this_cpu; > > > > + if (!sync && available_idle_cpu(prev_cpu)) > > + return prev_cpu; > > + > > this is not useful because when prev_cpu is idle, its runnable_avg was > null so the only > way for this_cpu to be selected by wake_affine_weight is to be null > too which is not really > possible when sync is set because sync is used to say, the running > task on this cpu > is about to sleep OK, I agree. Previously prev_eff_load was 0 when prev was idle, and whether the sync code is executed in wake_affine_weight or not, it will not b the case that this_eff_load < prev_eff_load, so this will not be selected. julia > > > return nr_cpumask_bits; > > } > > > > The sd->imbalance_pct part would have previously been a multiplication by > > 0, so it doesn't need to be taken into account. > > > > julia > > > > > > > > > The problem seems impossible to solve, because there is no way to know by > > > > looking only at prev and this whether the thread would prefer to stay > > > > where it was or go to the waker. > > > > > > > > > > Yes, this is definitely true. Looking at prev_cpu and this_cpu is a > > > crude approximation and the path is heavily limited in terms of how > > > clever it can be. > > > > > > -- > > > Mel Gorman > > > SUSE Labs > > > >
