Messages in this thread Patch in this message | | | Date | Fri, 25 Nov 2022 10:12:48 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v3] sched: async unthrottling for cfs bandwidth |
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On Fri, Nov 25, 2022 at 09:59:23AM +0100, Peter Zijlstra wrote: > On Fri, Nov 25, 2022 at 09:57:09AM +0100, Peter Zijlstra wrote: > > On Tue, Nov 22, 2022 at 11:35:48AM +0100, Peter Zijlstra wrote: > > > On Mon, Nov 21, 2022 at 11:37:14AM -0800, Josh Don wrote: > > > > Yep, this tradeoff feels "best", but there are some edge cases where > > > > this could potentially disrupt fairness. For example, if we have > > > > non-trivial W, a lot of cpus to iterate through for dispatching remote > > > > unthrottle, and quota is small. Doesn't help that the timer is pinned > > > > so that this will continually hit the same cpu. > > > > > > We could -- if we wanted to -- manually rotate the timer around the > > > relevant CPUs. Doing that sanely would require a bit of hrtimer surgery > > > though I'm afraid. > > > > Here; something like so should enable us to cycle the bandwidth timer. > > Just need to figure out a way to find another CPU or something. > > Some more preparation...
And then I think something like so.. That migrates the timer to the CPU of the first throttled entry -- possibly not the best heuristic, but its the simplest.
NOTE: none of this has seen a compiler up close.
--- --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5595,13 +5595,21 @@ static bool distribute_cfs_runtime(struc */ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun, unsigned long flags) { - int throttled; + struct cfs_rq *first_cfs_rq; + int throttled = 0; + int cpu; /* no need to continue the timer with no bandwidth constraint */ if (cfs_b->quota == RUNTIME_INF) goto out_deactivate; - throttled = !list_empty(&cfs_b->throttled_cfs_rq); + first_cfs_rq = list_first_entry_or_null(&cfs_b->throttled_cfs_rq, + struct cfs_rq, throttled_list); + if (first_cfs_rq) { + throttled = 1; + cpu = cpu_of(rq_of(first_cfs_rq)); + } + cfs_b->nr_periods += overrun; /* Refill extra burst quota even if cfs_b->idle */ @@ -5641,7 +5649,7 @@ static int do_sched_cfs_period_timer(str */ cfs_b->idle = 0; - return HRTIMER_RESTART; + return HRTIMER_RESTART_MIGRATE + cpu; out_deactivate: return HRTIMER_NORESTART;
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