Messages in this thread |  | | Subject | Re: [PATCH] sched/fair: handle case of task_h_load() returning 0 | From | Dietmar Eggemann <> | Date | Thu, 9 Jul 2020 15:34:50 +0200 |
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On 08/07/2020 11:47, Vincent Guittot wrote: > On Wed, 8 Jul 2020 at 11:45, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote: >> >> On 02/07/2020 16:42, Vincent Guittot wrote: >>> task_h_load() can return 0 in some situations like running stress-ng >>> mmapfork, which forks thousands of threads, in a sched group on a 224 cores >>> system. The load balance doesn't handle this correctly because >> >> I guess the issue here is that 'cfs_rq->h_load' in >> >> task_h_load() { >> struct cfs_rq *cfs_rq = task_cfs_rq(p); >> ... >> return div64_ul(p->se.avg.load_avg * cfs_rq->h_load, >> cfs_rq_load_avg(cfs_rq) + 1); >> } >> >> is still ~0 (or at least pretty small) compared to se.avg.load_avg being >> 1024 and cfs_rq_load_avg(cfs_rq) n*1024 in these lb occurrences. >> >>> env->imbalance never decreases and it will stop pulling tasks only after >>> reaching loop_max, which can be equal to the number of running tasks of >>> the cfs. Make sure that imbalance will be decreased by at least 1.
Looks like it's bounded by sched_nr_migrate (32 on my E5-2690 v2).
env.loop_max = min(sysctl_sched_nr_migrate, busiest->nr_running);
[...]
>> I assume that this is related to the LKP mail > > I have found this problem while studying the regression raised in the > email below but it doesn't fix it. At least, it's not enough > >> >> https://lkml.kernel.org/r/20200421004749.GC26573@shao2-debian ?
I see. It also happens with other workloads but it's most visible at the beginning of a workload (fork).
Still on E5-2690 v2 (2*2*10, 40 CPUs):
In the taskgroup cfs_rq->h_load is ~ 1024/40 = 25 so this leads to task_h_load = 0 with cfs_rq->avg.load_avg 40 times higher than the individual task load (1024).
One incarnation of 20 loops w/o any progress (that's w/o your patch).
With loop='loop/loop_break/loop_max' and load='p->se.avg.load_avg/cfs_rq->h_load/cfs_rq->avg.load_avg'
Jul 9 10:41:18 e105613-lin kernel: [73.068844] [stress-ng-mmapf 2907] SMT CPU37->CPU17 imb=8 loop=1/32/32 load=1023/23/43006 Jul 9 10:41:18 e105613-lin kernel: [73.068873] [stress-ng-mmapf 3501] SMT CPU37->CPU17 imb=8 loop=2/32/32 load=1022/23/41983 Jul 9 10:41:18 e105613-lin kernel: [73.068890] [stress-ng-mmapf 2602] SMT CPU37->CPU17 imb=8 loop=3/32/32 load=1023/23/40960 ... Jul 9 10:41:18 e105613-lin kernel: [73.069136] [stress-ng-mmapf 2520] SMT CPU37->CPU17 imb=8 loop=18/32/32 load=1023/23/25613 Jul 9 10:41:18 e105613-lin kernel: [73.069144] [stress-ng-mmapf 3107] SMT CPU37->CPU17 imb=8 loop=19/32/32 load=1021/23/24589 Jul 9 10:41:18 e105613-lin kernel: [73.069149] [stress-ng-mmapf 2672] SMT CPU37->CPU17 imb=8 loop=20/32/32 load=1024/23/23566 ...
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com> Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
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