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Date: Mon, 26 Oct 2020 16:42:14 +0100 From: Vincent Guittot <vincent.guittot@...aro.org> To: Rik van Riel <riel@...riel.com> Cc: Chris Mason <clm@...com>, Peter Zijlstra <peterz@...radead.org>, Johannes Weiner <hannes@...xchg.org>, linux-kernel <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] fix scheduler regression from "sched/fair: Rework load_balance()" On Mon, 26 Oct 2020 at 16:04, Rik van Riel <riel@...riel.com> wrote: > > On Mon, 2020-10-26 at 15:56 +0100, Vincent Guittot wrote: > > On Mon, 26 Oct 2020 at 15:38, Rik van Riel <riel@...riel.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@...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 > > Could utilization estimates be off, either lagging or > simply having a wrong estimate for a task, resulting > in no task getting pulled sometimes, while doing a > migrate_task imbalance always moves over something? task and cpu utilization are not always up to fully synced and may lag a bit which explains that sometimes LB can fail to migrate for a small diff > > Within an LLC we might not need to worry too much > about spare capacity, considering select_idle_sibling > doesn't give a hoot about capacity, either. > > -- > All Rights Reversed.
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