Messages in this thread | | | Date | Mon, 8 Apr 2024 11:06:39 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC][PATCH 08/10] sched/fair: Implement delayed dequeue |
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On Sat, Apr 06, 2024 at 05:23:25PM +0800, Chen Yu wrote:
> The 99th wakeup latency increases a little bit, and should be in the acceptible > range(25 -> 31 us).
Ah, my test runs haven't been stable enough to observe that.
> Meanwhile the throughput increases accordingly. Here are > the possible reason I can think of: > > 1. wakeup latency: The time to find an eligible entity in the tree > during wakeup might take longer - if there are more delayed-dequeue > tasks in the tree.
Another possible cause might be that previously a schedule() would be 1 dequeue, 1 pick.
But now it can be much more variable, a pick can basically do N dequeues and N+1 picks.
So not only do we do more picks, but if you're focussed on worst case latency, it goes up, because we can do multiple dequeues for a single pick.
If we find this to really be a problem, I had some half baked ideas to fix it, but it added significant complexity, so keep it simple until need proves we need more etc.
> 2. throughput: Inhibit task dequeue can decrease the ratio to touch the > task group's load_avg: dequeue_entity()-> { update_load_avg(), update_cfs_group()), > which reduces the cache contention among CPUs, and improves throughput.
Ah, yes, there's that.
> > + } else { > > + bool sleep = flags & DEQUEUE_SLEEP; > > + > > + SCHED_WARN_ON(sleep && se->sched_delayed); > > + update_curr(cfs_rq); > > + > > + if (sched_feat(DELAY_DEQUEUE) && sleep && > > + !entity_eligible(cfs_rq, se)) { > > Regarding the elibigle check, it was found that there could be an overflow > issue, and it brings false negative of entity_eligible(), which was described here: > https://lore.kernel.org/lkml/20240226082349.302363-1-yu.c.chen@intel.com/ > and also reported on another machine > https://lore.kernel.org/lkml/ZeCo7STWxq+oyN2U@gmail.com/ > I don't have good idea to avoid that overflow properly, while I'm trying to > reproduce it locally, do you have any guidance on how to address it?
I have not yet seen those, let me go stare at them now. Thanks!
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