Messages in this thread | | | Date | Thu, 8 Jun 2023 08:51:54 -0400 | From | Phil Auld <> | Subject | Re: [PATCH RESEND] sched/nohz: Add HRTICK_BW for using cfs bandwidth with nohz_full |
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Hi Ben,
On Thu, May 18, 2023 at 02:29:04PM -0700 Benjamin Segall wrote: > Phil Auld <pauld@redhat.com> writes: > > > CFS bandwidth limits and NOHZ full don't play well together. Tasks > > can easily run well past their quotas before a remote tick does > > accounting. This leads to long, multi-period stalls before such > > tasks can run again. Use the hrtick mechanism to set a sched > > tick to fire at remaining_runtime in the future if we are on > > a nohz full cpu, if the task has quota and if we are likely to > > disable the tick (nr_running == 1). This allows for bandwidth > > accounting before tasks go too far over quota. > > > > A number of container workloads use a dynamic number of real > > nohz tasks but also have other work that is limited which ends > > up running on the "spare" nohz cpus. This is an artifact of > > having to specify nohz_full cpus at boot. Adding this hrtick > > resolves the issue of long stalls on these tasks. > > > > Add the sched_feat HRTICK_BW off by default to allow users to > > enable this only when needed. > > > > Signed-off-by: Phil Auld <pauld@redhat.com> > > Suggested-by: Juri Lelli <jlelli@redhat.com> > > Cc: Ingo Molnar <mingo@redhat.com> > > Cc: Peter Zijlstra <peterz@infradead.org> > > Cc: Vincent Guittot <vincent.guittot@linaro.org> > > Cc: Juri Lelli <juri.lelli@redhat.com> > > Cc: Dietmar Eggemann <dietmar.eggemann@arm.com> > > Cc: Valentin Schneider <vschneid@redhat.com> > > Cc: Ben Segall <bsegall@google.com> > > --- > > > > Resend with LKML address instead of rh list... > > > > kernel/sched/core.c | 2 +- > > kernel/sched/fair.c | 20 ++++++++++++++++++++ > > kernel/sched/features.h | 1 + > > kernel/sched/sched.h | 12 ++++++++++++ > > 4 files changed, 34 insertions(+), 1 deletion(-) > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > index a68d1276bab0..76425c377245 100644 > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -6562,7 +6562,7 @@ static void __sched notrace __schedule(unsigned int sched_mode) > > > > schedule_debug(prev, !!sched_mode); > > > > - if (sched_feat(HRTICK) || sched_feat(HRTICK_DL)) > > + if (sched_feat(HRTICK) || sched_feat(HRTICK_DL) || sched_feat(HRTICK_BW)) > > hrtick_clear(rq); > > > > local_irq_disable(); > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index 373ff5f55884..0dd1f6a874bc 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -5309,6 +5309,22 @@ static int assign_cfs_rq_runtime(struct cfs_rq *cfs_rq) > > return ret; > > } > > > > +#if defined(CONFIG_SCHED_HRTICK) && defined(CONFIG_NO_HZ_FULL) > > +static void start_hrtick_cfs_bw(struct rq *rq, struct cfs_rq *cfs_rq) > > +{ > > + if (!tick_nohz_full_cpu(rq->cpu) || !cfs_bandwidth_used() || !cfs_rq->runtime_enabled) > > + return; > > + > > + /* runtime_remaining should never be negative here but just in case */ > > + if (rq->nr_running == 1 && cfs_rq->runtime_remaining > 0) > > + hrtick_start(rq, cfs_rq->runtime_remaining); > > +} > > +#else > > +static void start_hrtick_cfs_bw(struct rq *rq, struct cfs_rq *cfs_rq) > > +{ > > +} > > +#endif > > + > > static void __account_cfs_rq_runtime(struct cfs_rq *cfs_rq, u64 delta_exec) > > { > > /* dock delta_exec before expiring quota (as it could span periods) */ > > @@ -5481,6 +5497,7 @@ static bool throttle_cfs_rq(struct cfs_rq *cfs_rq) > > */ > > cfs_rq->throttled = 1; > > cfs_rq->throttled_clock = rq_clock(rq); > > + > > return true; > > } > > > > @@ -8096,6 +8113,9 @@ done: __maybe_unused; > > if (hrtick_enabled_fair(rq)) > > hrtick_start_fair(rq, p); > > > > + if (hrtick_enabled_bw(rq)) > > + start_hrtick_cfs_bw(rq, task_cfs_rq(p)); > > + > > update_misfit_status(p, rq); > > Implementation-wise this winds up with a tick of > sysctl_sched_cfs_bandwidth_slice, which I suppose the admin could _also_ > configure depending on how much NOHZ benefit vs cfsb issues they want. > > It's not great that this implementation winds up going all the way > through schedule() for each 5ms-default tick, though.O
I'll post my v2 shorly which adds a check to start the timer in the account_cfs_runtime() path when more runtime is allocated. This keeps the task running.
Since it takes a bit of time from the timer firing to acounting the task gets throttled each time. This ends up with a really short timer followed by a ~5ms one and repeating, due to the unthrottle code giving a runtime of 1.
I have a different approach which I think may be better, which is to prevent the tick_stop with a sched dependency when we have a single bw limited task. But I wanted to see what people think of this first approach now that it is fixed.
Thanks for taking a look.
Cheers, Phil
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