Messages in this thread | | | Subject | Re: [PATCH V3 2/2] sched/fair: Fallback to sched-idle CPU if idle CPU isn't found | From | Subhra Mazumdar <> | Date | Mon, 1 Jul 2019 15:08:41 -0700 |
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On 7/1/19 1:03 AM, Viresh Kumar wrote: > On 28-06-19, 18:16, Subhra Mazumdar wrote: >> On 6/25/19 10:06 PM, Viresh Kumar wrote: >>> We try to find an idle CPU to run the next task, but in case we don't >>> find an idle CPU it is better to pick a CPU which will run the task the >>> soonest, for performance reason. >>> >>> A CPU which isn't idle but has only SCHED_IDLE activity queued on it >>> should be a good target based on this criteria as any normal fair task >>> will most likely preempt the currently running SCHED_IDLE task >>> immediately. In fact, choosing a SCHED_IDLE CPU over a fully idle one >>> shall give better results as it should be able to run the task sooner >>> than an idle CPU (which requires to be woken up from an idle state). >>> >>> This patch updates both fast and slow paths with this optimization. >>> >>> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org> >>> --- >>> kernel/sched/fair.c | 43 +++++++++++++++++++++++++++++++++---------- >>> 1 file changed, 33 insertions(+), 10 deletions(-) >>> >>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >>> index 1277adc3e7ed..2e0527fd468c 100644 >>> --- a/kernel/sched/fair.c >>> +++ b/kernel/sched/fair.c >>> @@ -5376,6 +5376,15 @@ static struct { >>> #endif /* CONFIG_NO_HZ_COMMON */ >>> +/* CPU only has SCHED_IDLE tasks enqueued */ >>> +static int sched_idle_cpu(int cpu) >>> +{ >>> + struct rq *rq = cpu_rq(cpu); >>> + >>> + return unlikely(rq->nr_running == rq->cfs.idle_h_nr_running && >>> + rq->nr_running); >>> +} >>> + >> Shouldn't this check if rq->curr is also sched idle? > Why wouldn't the current set of checks be enough to guarantee that ? I thought nr_running does not include the on-cpu thread. > >> And why not drop the rq->nr_running non zero check? > Because CPU isn't sched-idle if nr_running and idle_h_nr_running are both 0, > i.e. it is an IDLE cpu in that case. And so I thought it is important to have > this check as well. > idle_cpu() not only checks nr_running is 0 but also rq->curr == rq->idle
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