Messages in this thread Patch in this message | | | Date | Fri, 10 Mar 2023 15:29:33 +0100 | From | Vincent Guittot <> | Subject | Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated |
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Le jeudi 09 mars 2023 à 16:14:38 (+0100), Vincent Guittot a écrit : > On Thu, 9 Mar 2023 at 15:37, Peter Zijlstra <peterz@infradead.org> wrote: > > > > On Thu, Mar 09, 2023 at 03:28:25PM +0100, Peter Zijlstra wrote: > > > On Thu, Mar 09, 2023 at 02:34:05PM +0100, Vincent Guittot wrote: > > > > > > > Then, even if we don't clear exec_start before migrating and keep > > > > current value to be used in place_entity on the new cpu, we can't > > > > compare the rq_clock_task(rq_of(cfs_rq)) of 2 different rqs AFAICT > > > > > > Blergh -- indeed, irq and steal time can skew them between CPUs :/ > > > I suppose we can fudge that... wait_start (which is basically what we're > > > making it do) also does that IIRC. > > > > > > I really dislike having this placement muck spreadout like proposed. > > > > Also, I think we might be over-engineering this, we don't care about > > accuracy at all, all we really care about is 'long-time'. > > you mean taking the patch 1/2 that you mentioned here to add a > migrated field: > https://lore.kernel.org/all/68832dfbb60fda030540b5f4e39c5801942689b1.1648228023.git.tim.c.chen@linux.intel.com/T/#ma5637eb8010f3f4a4abff778af8db705429d003b > > And assume that the divergence between the rq_clock_task() can be ignored ? > > That could probably work but we need to replace the (60LL * > NSEC_PER_SEC) by ((1ULL << 63) / NICE_0_LOAD) because 60sec divergence > would not be unrealistic. > and a comment to explain why it's acceptable
Zhang,
Could you try the patch below ? This is a rebase/merge/update of: -patch 1/2 above and -https://lore.kernel.org/lkml/20230209193107.1432770-1-rkagan@amazon.de/
The proposal accepts a divergence of up to 52 days between the 2 rqs.
If this work, we will prepare a proper patch
diff --git a/include/linux/sched.h b/include/linux/sched.h index 63d242164b1a..cb8af0a137f7 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -550,6 +550,7 @@ struct sched_entity { struct rb_node run_node; struct list_head group_node; unsigned int on_rq; + unsigned int migrated;
u64 exec_start; u64 sum_exec_runtime; diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 7a1b1f855b96..36acd9598b40 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -1057,6 +1057,7 @@ update_stats_curr_start(struct cfs_rq *cfs_rq, struct sched_entity *se) /* * We are starting a new run period: */ + se->migrated = 0; se->exec_start = rq_clock_task(rq_of(cfs_rq)); }
@@ -4684,13 +4685,23 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
/* * Pull vruntime of the entity being placed to the base level of - * cfs_rq, to prevent boosting it if placed backwards. If the entity - * slept for a long time, don't even try to compare its vruntime with - * the base as it may be too far off and the comparison may get - * inversed due to s64 overflow. + * cfs_rq, to prevent boosting it if placed backwards. + * However, min_vruntime can advance much faster than real time, with + * the exterme being when an entity with the minimal weight always runs + * on the cfs_rq. If the new entity slept for long, its vruntime + * difference from min_vruntime may overflow s64 and their comparison + * may get inversed, so ignore the entity's original vruntime in that + * case. + * The maximal vruntime speedup is given by the ratio of normal to + * minimal weight: NICE_0_LOAD / MIN_SHARES, so cutting off on the + * sleep time of 2^63 / NICE_0_LOAD should be safe. + * When placing a migrated waking entity, its exec_start has been set + * from a different rq. In order to take into account a possible + * divergence between new and prev rq's clocks task because of irq and + * stolen time, we take an additional margin. */ sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start; - if ((s64)sleep_time > 60LL * NSEC_PER_SEC) + if ((s64)sleep_time > (1ULL << 63) / NICE_0_LOAD / 2) se->vruntime = vruntime; else se->vruntime = max_vruntime(se->vruntime, vruntime); @@ -7658,7 +7669,7 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu) se->avg.last_update_time = 0;
/* We have migrated, no longer consider this task hot */ - se->exec_start = 0; + se->migrated = 1;
update_scan_period(p, new_cpu); } @@ -8344,6 +8355,9 @@ static int task_hot(struct task_struct *p, struct lb_env *env) if (sysctl_sched_migration_cost == 0) return 0;
+ if (p->se.migrated) + return 0; + delta = rq_clock_task(env->src_rq) - p->se.exec_start;
return delta < (s64)sysctl_sched_migration_cost;
> > > > > >
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