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 | Tue, 2 Jul 2019 09:32:27 -0700 |
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On 7/2/19 1:35 AM, Peter Zijlstra wrote: > On Mon, Jul 01, 2019 at 03:08:41PM -0700, Subhra Mazumdar wrote: >> 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: >>>>> @@ -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. > It very much does. > >>>> 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 > idle_cpu() will try very hard to declare a CPU !idle. But I don't see > how that it relevant. sched_idle_cpu() will only return true if there > are only SCHED_IDLE tasks on the CPU. Viresh's test is simple and > straight forward.
OK makes sense.
Thanks, Subhra
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