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SubjectRe: [PATCH v4 1/1] sched/fair: Return all runtime when cfs_b has very little remaining.
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Dave Chiluk <chiluk+linux@indeed.com> writes:

> It has been observed, that highly-threaded, user-interactive
> applications running under cpu.cfs_quota_us constraints can hit a high
> percentage of periods throttled while simultaneously not consuming the
> allocated amount of quota. This impacts user-interactive non-cpu bound
> applications, such as those running in kubernetes or mesos when run on
> multiple cores.
>
> This has been root caused to threads being allocated per cpu bandwidth
> slices, and then not fully using that slice within the period. This
> results in min_cfs_rq_runtime remaining on each per-cpu cfs_rq. At the
> end of the period this remaining quota goes unused and expires. This
> expiration of unused time on per-cpu runqueues results in applications
> under-utilizing their quota while simultaneously hitting throttling.
>
> The solution is to return all spare cfs_rq->runtime_remaining when
> cfs_b->runtime nears the sched_cfs_bandwidth_slice. This balances the
> desire to prevent cfs_rq from always pulling quota with the desire to
> allow applications to fully utilize their quota.
>
> Fixes: 512ac999d275 ("sched/fair: Fix bandwidth timer clock drift condition")
> Signed-off-by: Dave Chiluk <chiluk+linux@indeed.com>
> ---
> kernel/sched/fair.c | 19 ++++++++++++++++---
> 1 file changed, 16 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f35930f..4894eda 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4695,7 +4695,9 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun, u
> return 1;
> }
>
> -/* a cfs_rq won't donate quota below this amount */
> +/* a cfs_rq won't donate quota below this amount unless cfs_b has very little
> + * remaining runtime.
> + */
> static const u64 min_cfs_rq_runtime = 1 * NSEC_PER_MSEC;
> /* minimum remaining period time to redistribute slack quota */
> static const u64 min_bandwidth_expiration = 2 * NSEC_PER_MSEC;
> @@ -4743,16 +4745,27 @@ static void start_cfs_slack_bandwidth(struct cfs_bandwidth *cfs_b)
> static void __return_cfs_rq_runtime(struct cfs_rq *cfs_rq)
> {
> struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(cfs_rq->tg);
> - s64 slack_runtime = cfs_rq->runtime_remaining - min_cfs_rq_runtime;
> + s64 slack_runtime = cfs_rq->runtime_remaining;
>
> + /* There is no runtime to return. */
> if (slack_runtime <= 0)
> return;
>
> raw_spin_lock(&cfs_b->lock);
> if (cfs_b->quota != RUNTIME_INF &&
> cfs_rq->runtime_expires == cfs_b->runtime_expires) {
> - cfs_b->runtime += slack_runtime;
> + /* As we near 0 quota remaining on cfs_b start returning all
> + * remaining runtime. This avoids stranding and then expiring
> + * runtime on per-cpu cfs_rq.
> + *
> + * cfs->b has plenty of runtime leave min_cfs_rq_runtime of
> + * runtime on this cfs_rq.
> + */
> + if (cfs_b->runtime >= sched_cfs_bandwidth_slice() * 3 &&
> + slack_runtime > min_cfs_rq_runtime)
> + slack_runtime -= min_cfs_rq_runtime;
>
> + cfs_b->runtime += slack_runtime;
> /* we are under rq->lock, defer unthrottling using a timer */
> if (cfs_b->runtime > sched_cfs_bandwidth_slice() &&
> !list_empty(&cfs_b->throttled_cfs_rq))


This still has a similar cost as reducing min_cfs_rq_runtime to 0 - we
now take a tg-global lock on every group se dequeue. Setting min=0 means
that we have to take it on both enqueue and dequeue, while baseline
means we take it once per min_cfs_rq_runtime in the worst case.

In addition how much this helps is very dependent on the exact pattern
of sleep/wake - you can still strand all but 15ms of runtime with a
pretty reasonable pattern.

If the cost of taking this global lock across all cpus without a
ratelimit was somehow not a problem, I'd much prefer to just set
min_cfs_rq_runtime = 0. (Assuming it is, I definitely prefer the "lie
and sorta have 2x period 2x runtime" solution of removing expiration)

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