Re: [PATCH v4 1/1] sched/fair: Return all runtime when cfs_b has very little remaining.

From: Dave Chiluk
Date: Wed Jun 26 2019 - 18:10:32 EST

On Mon, Jun 24, 2019 at 10:33:07AM -0700, bsegall@xxxxxxxxxx wrote:
> 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.
Yep, it's only slightly better than simply setting min_cfs_rq_runtime=0.
There's definitely a tradeoff of having to grab the lock every time.

The other non-obvious benefit to this is that when the application
starts hitting throttling the entire application starts hitting
throttling closer to simultaneously. Previously, threads that don't do
much could continue sipping on their 1ms of min_cfs_rq_runtime, while
main threads were throttled. With this the application would more or
less halt within 5ms of full quota usage.

> 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.

I thought this change would have an upper bound stranded time of:
NUMCPUs * min_cfs_rq_runtime - 3 * sched_cfs_bandwidth_slice().
>From the stranding of 1ms on each queue minues the amount that we'd
return when we forcibly hit the 3 x bandwidth slice Am I missing
something here? Additionally that's worst case, and would require that
threads be scheduled on distinct cpus mostly sequentially, and then
never run again.

> 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)

Can I take this as an technical ack of the v3 patchset? Do you have any
other comments for that patchset you'd like to see corrected before it
gets integrated? If you are indeed good with that patchset, I'll clean
up the comment and Documentation text and re-submit as a v5 patchset. I

actually like that patchset best as well, for all the reasons already
discussed. Especially the one where it's been running like this for 5
years prior to 512ac999.

Also given my new-found understanding of min_cfs_rq_runtime, doesn't
this mean that if we don't expire runtime we could potentially have a
worst case of 1ms * NUMCPUs of extra runtime instead of 2x runtime?
This is because min_cfs_rq_runtime could theoretically live on a per-cpu
indefinitely? Still this is actually much smaller than I previously had
feared. Also it still holds that average runtime is still bounded by
runtime/period on average.