Messages in this thread | | | Date | Fri, 22 Sep 2023 10:13:11 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH v2] sched: fix warning in bandwidth distribution |
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* Josh Don <joshdon@google.com> wrote:
> We've observed the following warning being hit in > distribute_cfs_runtime(): > SCHED_WARN_ON(cfs_rq->runtime_remaining > 0) > > We have the following race: > > - cpu0: running bandwidth distribution (distribute_cfs_runtime). > Inspects the local cfs_rq and makes its runtime_remaining positive. > However, we defer unthrottling the local cfs_rq until after > considering all remote cfs_rq's. > - cpu1: starts running bandwidth distribution from the slack timer. When > it finds the cfs_rq for cpu 0 on the throttled list, it observers the > that the cfs_rq is throttled, yet is not on the CSD list, and has a > positive runtime_remaining, thus triggering the warning in > distribute_cfs_runtime. > > To fix this, we can rework the local unthrottling logic to put the local > cfs_rq on a local list, so that any future bandwidth distributions will > realize that the cfs_rq is about to be unthrottled. > > Signed-off-by: Josh Don <joshdon@google.com> > --- > v2: Fix build error on !CONFIG_SMP > > kernel/sched/fair.c | 43 +++++++++++++++++++++++++++++++++---------- > 1 file changed, 33 insertions(+), 10 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 384900bf87eb..3d1886ea18fe 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -5743,13 +5743,16 @@ static void unthrottle_cfs_rq_async(struct cfs_rq *cfs_rq) > > static bool distribute_cfs_runtime(struct cfs_bandwidth *cfs_b) > { > - struct cfs_rq *local_unthrottle = NULL; > int this_cpu = smp_processor_id(); > u64 runtime, remaining = 1; > bool throttled = false; > struct cfs_rq *cfs_rq; > struct rq_flags rf; > struct rq *rq; > +#ifdef CONFIG_SMP > + struct cfs_rq *tmp; > + LIST_HEAD(local_unthrottle); > +#endif > > rcu_read_lock(); > list_for_each_entry_rcu(cfs_rq, &cfs_b->throttled_cfs_rq, > @@ -5786,11 +5789,21 @@ static bool distribute_cfs_runtime(struct cfs_bandwidth *cfs_b) > > /* we check whether we're throttled above */ > if (cfs_rq->runtime_remaining > 0) { > - if (cpu_of(rq) != this_cpu || > - SCHED_WARN_ON(local_unthrottle)) > +#ifdef CONFIG_SMP > + if (cpu_of(rq) != this_cpu) { > unthrottle_cfs_rq_async(cfs_rq); > - else > - local_unthrottle = cfs_rq; > + } else { > + /* > + * We currently only expect to be unthrottling > + * a single cfs_rq locally. > + */ > + SCHED_WARN_ON(!list_empty(&local_unthrottle)); > + list_add_tail(&cfs_rq->throttled_csd_list, > + &local_unthrottle); > + } > +#else > + unthrottle_cfs_rq_async(cfs_rq); > +#endif > } else { > throttled = true; > } > @@ -5798,15 +5811,25 @@ static bool distribute_cfs_runtime(struct cfs_bandwidth *cfs_b) > next: > rq_unlock_irqrestore(rq, &rf); > } > - rcu_read_unlock(); > > - if (local_unthrottle) { > - rq = cpu_rq(this_cpu); > +#ifdef CONFIG_SMP > + list_for_each_entry_safe(cfs_rq, tmp, &local_unthrottle, > + throttled_csd_list) { > + struct rq *rq = rq_of(cfs_rq); > + > rq_lock_irqsave(rq, &rf); > - if (cfs_rq_throttled(local_unthrottle)) > - unthrottle_cfs_rq(local_unthrottle); > + > + list_del_init(&cfs_rq->throttled_csd_list); > + > + if (cfs_rq_throttled(cfs_rq)) > + unthrottle_cfs_rq(cfs_rq); > + > rq_unlock_irqrestore(rq, &rf); > } > + SCHED_WARN_ON(!list_empty(&local_unthrottle)); > +#endif
So instead of uglifying the code with seldom-tested !CONFIG_SMP #ifdefs, please fold the !SMP code into SMP code: make ->throttled_csd_list unconditional in a preparatory patch (even if it's essentially unused on !SMP), then add what is basically your v1 patch as a second patch in the series to fix the bug.
We want to unify as much of the SMP and !SMP codepaths as possible, even when it casuses a bit of runtime overhead for !SMP.
Thanks,
Ingo
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