Messages in this thread | | | Date | Tue, 1 Aug 2023 17:37:31 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v6 2/2] Sched/fair: Block nohz tick_stop when cfs bandwidth in use |
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On Tue, Aug 01, 2023 at 07:13:42AM -0400, Phil Auld wrote: > On Tue, Aug 01, 2023 at 12:49:34AM +0200 Peter Zijlstra wrote: > > On Wed, Jul 12, 2023 at 09:33:57AM -0400, Phil Auld wrote: > > > 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. Currently, when presented with these conflicting > > > requirements the scheduler is favoring nohz_full and letting the tick > > > be stopped. However, nohz tick stopping is already best-effort, there > > > are a number of conditions that can prevent it, whereas cfs runtime > > > bandwidth is expected to be enforced. > > > > > > Make the scheduler favor bandwidth over stopping the tick by setting > > > TICK_DEP_BIT_SCHED when the only running task is a cfs task with > > > runtime limit enabled. We use cfs_b->hierarchical_quota to > > > determine if the task requires the tick. > > > > > > Add check in pick_next_task_fair() as well since that is where > > > we have a handle on the task that is actually going to be running. > > > > > > Add check in sched_can_stop_tick() to cover some edge cases such > > > as nr_running going from 2->1 and the 1 remains the running task. > > > > These appear fine to me, except: > > > > > Add sched_feat HZ_BW (off by default) to control the tick_stop > > > behavior. > > > > What was the thinking here? This means nobody will be using this -- why > > would you want this default disabled? > > > > That was just a hedge in case it caused issues. I'd probably have had to > enable it in RHEL anyway. Using a feature was to make it inocuous when > disabled. Would you prefer me to enable it or remove the sched_feat > entirely? (or do you want to just switch that to true when you apply it?)
I've edited it to default enabled -- we can pull the feature flag eventually I suppose.
Things didn't readily apply, so I've kicked at it a little. Should be in queue/sched/core for the robots to chew on.
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