Messages in this thread | | | From | Vincent Guittot <> | Date | Mon, 26 Oct 2020 15:56:58 +0100 | Subject | Re: [PATCH] fix scheduler regression from "sched/fair: Rework load_balance()" |
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On Mon, 26 Oct 2020 at 15:38, Rik van Riel <riel@surriel.com> wrote: > > On Mon, 2020-10-26 at 15:24 +0100, Vincent Guittot wrote: > > Le lundi 26 oct. 2020 à 08:45:27 (-0400), Chris Mason a écrit : > > > On 26 Oct 2020, at 4:39, Vincent Guittot wrote: > > > > > > > Hi Chris > > > > > > > > On Sat, 24 Oct 2020 at 01:49, Chris Mason <clm@fb.com> wrote: > > > > > Hi everyone, > > > > > > > > > > We’re validating a new kernel in the fleet, and compared with > > > > > v5.2, > > > > > > > > Which version are you using ? > > > > several improvements have been added since v5.5 and the rework of > > > > load_balance > > > > > > We’re validating v5.6, but all of the numbers referenced in this > > > patch are > > > against v5.9. I usually try to back port my way to victory on this > > > kind of > > > thing, but mainline seems to behave exactly the same as > > > 0b0695f2b34a wrt > > > this benchmark. > > > > ok. Thanks for the confirmation > > > > I have been able to reproduce the problem on my setup. > > > > Could you try the fix below ? > > > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -9049,7 +9049,8 @@ static inline void calculate_imbalance(struct > > lb_env *env, struct sd_lb_stats *s > > * emptying busiest. > > */ > > if (local->group_type == group_has_spare) { > > - if (busiest->group_type > group_fully_busy) { > > + if ((busiest->group_type > group_fully_busy) && > > + (busiest->group_weight > 1)) { > > /* > > * If busiest is overloaded, try to fill > > spare > > * capacity. This might end up creating spare > > capacity > > > > > > When we calculate an imbalance at te smallest level, ie between CPUs > > (group_weight == 1), > > we should try to spread tasks on cpus instead of trying to fill spare > > capacity. > > Should we also spread tasks when balancing between > multi-threaded CPU cores on the same socket?
My explanation is probably misleading. In fact we already try to spread tasks. we just use spare capacity instead of nr_running when there is more than 1 CPU in the group and the group is overloaded. Using spare capacity is a bit more conservative because it tries to not pull more utilization than spare capacity
> > Say we have groups of CPUs > (0, 2) and CPUs (1, 3), > with CPU 2 idle, and 3 tasks spread between CPUs > 1 & 3. > > Since they are all on the same LLC, and the task > wakeup code has absolutely no hesitation in moving > them around, should the load balancer also try to > keep tasks within a socket spread across all CPUs? > > -- > All Rights Reversed.
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