Messages in this thread | | | From | Vincent Guittot <> | Date | Tue, 21 Feb 2023 10:38:44 +0100 | Subject | Re: [PATCH v3] sched/fair: sanitize vruntime of entity being placed |
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On Thu, 9 Feb 2023 at 20:31, Roman Kagan <rkagan@amazon.de> wrote: > > From: Zhang Qiao <zhangqiao22@huawei.com> > > When a scheduling entity is placed onto cfs_rq, its vruntime is pulled > to the base level (around cfs_rq->min_vruntime), so that the entity > doesn't gain extra boost when placed backwards. > > However, if the entity being placed wasn't executed for a long time, its > vruntime may get too far behind (e.g. while cfs_rq was executing a > low-weight hog), which can inverse the vruntime comparison due to s64 > overflow. This results in the entity being placed with its original > vruntime way forwards, so that it will effectively never get to the cpu. > > To prevent that, ignore the vruntime of the entity being placed if it > didn't execute for longer than the time that can lead to an overflow. > > Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com> > [rkagan: formatted, adjusted commit log, comments, cutoff value] > Co-developed-by: Roman Kagan <rkagan@amazon.de> > Signed-off-by: Roman Kagan <rkagan@amazon.de>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
> --- > v2 -> v3: > - make cutoff less arbitrary and update comments [Vincent] > > v1 -> v2: > - add Zhang Qiao's s-o-b > - fix constant promotion on 32bit > > kernel/sched/fair.c | 21 +++++++++++++++++++-- > 1 file changed, 19 insertions(+), 2 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 0f8736991427..3baa6b7ea860 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -4656,6 +4656,7 @@ static void > place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial) > { > u64 vruntime = cfs_rq->min_vruntime; > + u64 sleep_time; > > /* > * The 'current' period is already promised to the current tasks, > @@ -4685,8 +4686,24 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial) > vruntime -= thresh; > } > > - /* ensure we never gain time by being placed backwards. */ > - se->vruntime = max_vruntime(se->vruntime, vruntime); > + /* > + * Pull vruntime of the entity being placed to the base level of > + * 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. > + */ > + sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start; > + if ((s64)sleep_time > (1ULL << 63) / NICE_0_LOAD) > + se->vruntime = vruntime; > + else > + se->vruntime = max_vruntime(se->vruntime, vruntime); > } > > static void check_enqueue_throttle(struct cfs_rq *cfs_rq); > -- > 2.34.1 > > > > > Amazon Development Center Germany GmbH > Krausenstr. 38 > 10117 Berlin > Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss > Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B > Sitz: Berlin > Ust-ID: DE 289 237 879 > > >
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