linux-kernel - Re: [PATCH] fix scheduler regression from "sched/fair: Rework load

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Date:   Mon, 26 Oct 2020 12:48:30 -0400
From:   "Chris Mason" <clm@...com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
CC:     Rik van Riel <riel@...riel.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 26 Oct 2020, at 12:20, Vincent Guittot wrote:

> Le lundi 26 oct. 2020 à 12:04:45 (-0400), Rik van Riel a écrit :
>> On Mon, 26 Oct 2020 16:42:14 +0100
>> Vincent Guittot <vincent.guittot@...aro.org> wrote:
>>> On Mon, 26 Oct 2020 at 16:04, Rik van Riel <riel@...riel.com> wrote:
>>
>>>> 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
>>
>> OK, running with this little snippet below, I see latencies
>> improve back to near where they used to be:
>>
>> Latency percentiles (usec) runtime 150 (s)
>>         50.0th: 13
>>         75.0th: 31
>>         90.0th: 69
>>         95.0th: 90
>>         *99.0th: 761
>>         99.5th: 2268
>>         99.9th: 9104
>>         min=1, max=16158
>>
>> I suspect the right/cleaner approach might be to use
>> migrate_task more in !CPU_NOT_IDLE cases?
>>
>> Running a task to an idle CPU immediately, instead of refusing
>> to have the load balancer move it, improves latencies for fairly
>> obvious reasons.
>>
>> I am not entirely clear on why the load balancer should need to
>> be any more conservative about moving tasks than the wakeup
>> path is in eg. select_idle_sibling.
>
>
> what you are suggesting is something like:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 4978964e75e5..3b6fbf33abc2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9156,7 +9156,8 @@ 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) &&
> +                   !(env->sd->flags & SD_SHARE_PKG_RESOURCES)) {
>                         /*
>                          * If busiest is overloaded, try to fill spare
>                          * capacity. This might end up creating spare 
> capacity
>
> which also fixes the problem for me and alignes LB with wakeup path 
> regarding the migration
> in the LLC

Vincent’s patch on top of 5.10-rc1 looks pretty great:

Latency percentiles (usec) runtime 90 (s) (3320 total samples)
         50.0th: 161 (1687 samples)
         75.0th: 200 (817 samples)
         90.0th: 228 (488 samples)
         95.0th: 254 (164 samples)
         *99.0th: 314 (131 samples)
         99.5th: 330 (17 samples)
         99.9th: 356 (13 samples)
         min=29, max=358

Next we test in prod, which probably won’t have answers until 
tomorrow.  Thanks again Vincent!

-chris