Messages in this thread Patch in this message | ![/](/images/icornerl.gif) | | From | Vincent Guittot <> | Date | Mon, 13 Mar 2023 15:23:56 +0100 | Subject | Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated |
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On Sat, 11 Mar 2023 at 10:57, Zhang Qiao <zhangqiao22@huawei.com> wrote: > > > > 在 2023/3/10 22:29, Vincent Guittot 写道: > > 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/ > > > I applyed and tested this patch, and it make hackbench slower. > According to my previous test results. The good result is 82.1(s). > But the result of this patch is 108.725(s).
By "the result of this patch is 108.725(s)", you mean the result of https://lore.kernel.org/lkml/20230209193107.1432770-1-rkagan@amazon.de/ alone, don't you ?
> > > > version1: v6.2 > > version2: v6.2 + commit 829c1651e9c4 > > version3: v6.2 + commit 829c1651e9c4 + this patch > > > > ------------------------------------------------- > > version1 version2 version3 > > test1 81.0 118.1 82.1 > > test2 82.1 116.9 80.3 > > test3 83.2 103.9 83.3 > > avg(s) 82.1 113.0 81.9
Ok, it looks like we are back to normal figures
> > > > ------------------------------------------------- > > > > 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 > > why not is `scale_load_down(NICE_0_LOAD) / MIN_SHARES` here ?
yes, you are right.
> > > > + * 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 > > This divergence might be larger, it cause `sleep_time` maybe negative.
AFAICT, we are safe with ((1ULL << 63) / scale_load_down(NICE_0_LOAD) / 2) as long as the divergence between the 2 rqs clocks task is lower than 2^52nsec. Do you expect a divergence higher than 2^52 nsec (around 52 days)?
We can probably keep using (1ULL << 63) / scale_load_down(NICE_0_LOAD) which is already half the max value if needed.
the fact that sleep_time can be negative is not a problem as s64)sleep_time > will take care of this.
> > > + * 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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