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Date: Wed, 8 Jun 2022 19:55:15 -0700 From: Josh Don <joshdon@...gle.com> To: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org> Cc: Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, Daniel Bristot de Oliveira <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com>, linux-kernel@...r.kernel.org, Josh Don <joshdon@...gle.com> Subject: [PATCH] sched: allow newidle balancing to bail out of load_balance While doing newidle load balancing, it is possible for new tasks to arrive, such as with pending wakeups. newidle_balance() already accounts for this by exiting the sched_domain load_balance() iteration if it detects these cases. This is very important for minimizing wakeup latency. However, if we are already in load_balance(), we may stay there for a while before returning back to newidle_balance(). This is most exacerbated if we enter a 'goto redo' loop in the LBF_ALL_PINNED case. A very straightforward workaround to this is to adjust should_we_balance() to bail out if we're doing a CPU_NEWLY_IDLE balance and new tasks are detected. This was tested with the following reproduction: - two threads that take turns sleeping and waking each other up are affined to two cores - a large number of threads with 100% utilization are pinned to all other cores Without this patch, wakeup latency was ~120us for the pair of threads, almost entirely spent in load_balance(). With this patch, wakeup latency is ~6us. Signed-off-by: Josh Don <joshdon@...gle.com> --- kernel/sched/fair.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 8c5b74f66bd3..5abf30117824 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -9834,9 +9834,15 @@ static int should_we_balance(struct lb_env *env) /* * In the newly idle case, we will allow all the CPUs * to do the newly idle load balance. + * + * However, we bail out if we already have tasks or a wakeup pending, + * to optimize wakeup latency. */ - if (env->idle == CPU_NEWLY_IDLE) + if (env->idle == CPU_NEWLY_IDLE) { + if (env->dst_rq->nr_running > 0 || env->dst_rq->ttwu_pending) + return 0; return 1; + } /* Try to find first idle CPU */ for_each_cpu_and(cpu, group_balance_mask(sg), env->cpus) { -- 2.36.1.476.g0c4daa206d-goog
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