Messages in this thread Patch in this message | | | Date | Mon, 26 Oct 2020 16:18:46 +0100 | From | Vincent Guittot <> | Subject | Re: [PATCH] fix scheduler regression from "sched/fair: Rework load_balance()" |
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Le lundi 26 oct. 2020 à 11:05:35 (-0400), Chris Mason a écrit : > > > On 26 Oct 2020, at 10:24, 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. > > Thanks for taking a look! Can I ask what parameters you used on schbench, > and what kind of results you saw? Mostly I’m trying to make sure it’s a > useful tool, but also the patch didn’t change things here. >
with latest tip/sched/core on my dual quad cores: schbench -t 4 -r 10 -c 1000000 -s 1000 Latency percentiles (usec) 50.0th: 16 75.0th: 23 90.0th: 32 95.0th: 41 *99.0th: 15120 99.5th: 15120 99.9th: 15120 min=0, max=15130
with the patch : schbench -t 4 -r 10 -c 1000000 -s 1000 Latency percentiles (usec) 50.0th: 28 75.0th: 32 90.0th: 36 95.0th: 56 *99.0th: 1310 99.5th: 1310 99.9th: 1310 min=0, max=1309
> > > > 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. > > With this patch on top of v5.9, my latencies are unchanged. I’m building > against current Linus now just in case I’m missing other fixes. >
I can't remember any changes in mainline that would make a difference
I had another way to fix it but it could impact more other UC and the improvement was smaller
--- kernel/sched/fair.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index ebe15e36f336..415927885228 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7707,7 +7707,7 @@ static int detach_tasks(struct lb_env *env) case migrate_util: util = task_util_est(p);
- if (util > env->imbalance) + if ((util >> env->sd->nr_balance_failed) > env->imbalance) goto next;
env->imbalance -= util; --
> > -chris
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