Messages in this thread | | | Date | Wed, 23 Feb 2022 09:14:45 -0600 | Subject | Re: [PATCH] kernel/sched: Update schedstats when migrating threads | From | Carlos Bilbao <> |
| |
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).
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
| |