Messages in this thread | | | From | Josh Don <> | Date | Wed, 2 Nov 2022 17:10:08 -0700 | Subject | Re: [PATCH v2] sched: async unthrottling for cfs bandwidth |
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Hi Michal,
Thanks for taking a look.
On Wed, Nov 2, 2022 at 9:59 AM Michal Koutný <mkoutny@suse.com> wrote: > > Hello. > > On Wed, Oct 26, 2022 at 03:44:49PM -0700, Josh Don <joshdon@google.com> wrote: > > To fix this, we can instead unthrottle cfs_rq's asynchronously via a > > CSD. Each cpu is responsible for unthrottling itself, thus sharding the > > total work more fairly across the system, and avoiding hard lockups. > > FIFO behavior of the cfs_b->throttled_cfs_rq is quite important to > ensure fairness of throttling (historically when it FIFO wasn't honored, > it caused some cfs_rq starving issues). > > Despite its name, distribute_cfs_runtime() doesn't distribute the > runtime, the time is pulled inside assign_cfs_rq_runtime() (but that's > already on target cpu). > Currently, it's all synchronized under cfs_b->lock but with your change, > throttled cfs_rq would be dissolved among cpus that'd run concurrently > (assign_cfs_rq_runtime() still takes cfs_b->lock but it won't be > necessarily in the unthrottling order).
I don't think my patch meaningfully regresses this; the prior state was also very potentially unfair in a similar way.
Without my patch, distribute_cfs_runtime() will unthrottle the cfs_rq's, and as you point out, it doesn't actually give them any real quota, it lets assign_cfs_rq_runtime() take care of that. But this happens asynchronously on those cpus. If they are idle, they wait for an IPI from the resched_curr() in unthrottled_cfs_rq(), otherwise they simply wait until potentially the next rescheduling point. So we are currently far from ever being guaranteed that the order the cpus pull actual quota via assign_cfs_rq_runtime() matches the order they were unthrottled from the list.
> > +static inline void __unthrottle_cfs_rq_async(struct cfs_rq *cfs_rq) > > [...] > > + if (rq == this_rq()) { > > + unthrottle_cfs_rq(cfs_rq); > > + return; > > + } > > It was pointed out to me that generic_exec_single() does something > similar. > Wouldn't the flow bandwidth control code be simpler relying on that?
We already hold rq lock so we couldn't rely on the generic_exec_single() special case since that would double lock.
> Also, can a particular cfs_rq be on both cfs_b->throttled_csd_list and > cfs_b->throttled_cfs_rq lists at any moment? > I wonder if having a single list_head node in cfs_rq would be feasible > (and hence enforcing this constraint in data).
That's an interesting idea, this could be rewritten so that distribute() pulls the entity off this list and moves it to the throttled_csd_list; we never have an actual need to have entities on both lists at the same time.
I'll wait to see if Peter has any comments, but that could be made in a v3 for this patch.
Best, Josh
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