Messages in this thread | | | Date | Wed, 23 Feb 2022 09:33:59 -0600 | Subject | Re: [PATCH] kernel/sched: Update schedstats when migrating threads | From | Carlos Bilbao <> |
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On 2/23/2022 9:28 AM, Phil Auld wrote: > On Wed, Feb 23, 2022 at 09:14:45AM -0600 Carlos Bilbao wrote: >> On 2/23/2022 4:19 AM, Peter Zijlstra wrote: >>> On Wed, Jan 26, 2022 at 09:22:23AM -0600, Carlos Bilbao wrote: >>>> The kernel manages per-task scheduler statistics or schedstats. Such >>>> counters should be reinitialized when the thread is migrated to a >>>> different core rq, except for the values recording number of migrations. >>> >>> I'm confused, why should we reset schedstats on migrate? I'm thinking >>> this breaks per-task, since tasks tend to bounce around quite a lot. >>> >> >> Thanks for your comments, Peter. >> >> Looking at the documentation of schedstats I see that most values are >> actually linked to the particular CPU: time spent on the cpu, timeslices >> run on this cpu, number of times _something_ was called when the cpu was >> idle, and so forth. Those values lose their meaning after migration and we >> should reinitialize their counters. However, reviewing sched_statistics I >> identify two fields that we should definitely keep increasing even after >> migration (nr_migrations_cold, nr_forced_migrations). >> > > The documentation is a little off. I think it should say "any cpu" instead > of "this cpu". If you reset these per task counters (time on cpu, number > of timeslices etc) on every migration then they become meaningless (and > useless). > > > Cheers, > Phil >
Well that clarifies it! Then, let me ask the opposite question... What fields of schedstats should we clear when migrating? If there isn't any, I will just increase the number of migrations.
>> So this patch will have to be upgraded if there's some other value(s) in >> schedstats that we do not want to reinitialize either. >> >>>> Signed-off-by: Carlos Bilbao <carlos.bilbao@amd.com> >>>> --- >>>> kernel/sched/core.c | 10 +++++++++- >>>> 1 file changed, 9 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >>>> index fe53e510e711..d64c2a290176 100644 >>>> --- a/kernel/sched/core.c >>>> +++ b/kernel/sched/core.c >>>> @@ -8757,6 +8757,7 @@ bool sched_smp_initialized __read_mostly; >>>> int migrate_task_to(struct task_struct *p, int target_cpu) >>>> { >>>> struct migration_arg arg = { p, target_cpu }; >>>> + uint64_t forced_migrations, migrations_cold; >>>> int curr_cpu = task_cpu(p); >>>> >>>> if (curr_cpu == target_cpu) >>>> @@ -8765,7 +8766,14 @@ int migrate_task_to(struct task_struct *p, int target_cpu) >>>> if (!cpumask_test_cpu(target_cpu, p->cpus_ptr)) >>>> return -EINVAL; >>>> >>>> - /* TODO: This is not properly updating schedstats */ >>>> + if (schedstat_enabled()) { >>>> + forced_migrations = schedstat_val(p->stats.nr_forced_migrations); >>>> + migrations_cold = schedstat_val(p->stats.nr_migrations_cold); >>>> + memset(&p->stats, 0, sizeof(p->stats)); >>>> + schedstat_set(p->stats.nr_forced_migrations, forced_migrations); >>>> + schedstat_set(p->stats.nr_migrations_cold, migrations_cold); >>>> + schedstat_inc(p->stats.nr_migrations_cold); >>>> + } >>>> >>>> trace_sched_move_numa(p, curr_cpu, target_cpu); >>>> return stop_one_cpu(curr_cpu, migration_cpu_stop, &arg); >>>> -- >>>> 2.27.0 >>>> >> >> Thanks, >> Carlos >> >
Thanks, Carlos
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