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Date:   Sun, 28 Apr 2019 21:58:34 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Frederic Weisbecker <fweisbec@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] kernel/sched: run nohz idle load balancer on HK_FLAG_MISC
 CPUs

Wanpeng Li's on April 28, 2019 5:01 pm:
> On Fri, 12 Apr 2019 at 12:27, Nicholas Piggin <npiggin@...il.com> wrote:
>>
>> The nohz idle balancer runs on the lowest idle CPU. This can
>> interfere with isolated CPUs, so confine it to HK_FLAG_MISC
>> housekeeping CPUs.
>>
>> HK_FLAG_SCHED is not used for this because it is not set anywhere
>> at the moment. This could be folded into HK_FLAG_SCHED once that
>> option is fixed.
>>
>> The problem was observed with increased jitter on an application
>> running on CPU0, caused by nohz idle load balancing being run on
>> CPU1 (an SMT sibling).
>>
>> Signed-off-by: Nicholas Piggin <npiggin@...il.com>
>> ---
>>  kernel/sched/fair.c | 16 ++++++++++------
>>  1 file changed, 10 insertions(+), 6 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index fdab7eb6f351..d29ca323214d 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -9522,22 +9522,26 @@ static inline int on_null_domain(struct rq *rq)
>>   * - When one of the busy CPUs notice that there may be an idle rebalancing
>>   *   needed, they will kick the idle load balancer, which then does idle
>>   *   load balancing for all the idle CPUs.
>> + * - HK_FLAG_MISC CPUs are used for this task, because HK_FLAG_SCHED not set
>> + *   anywhere yet.
>>   */
>>
>>  static inline int find_new_ilb(void)
>>  {
>> -       int ilb = cpumask_first(nohz.idle_cpus_mask);
>> +       int ilb;
>>
>> -       if (ilb < nr_cpu_ids && idle_cpu(ilb))
>> -               return ilb;
>> +       for_each_cpu_and(ilb, nohz.idle_cpus_mask,
>> +                             housekeeping_cpumask(HK_FLAG_MISC)) {
>> +               if (idle_cpu(ilb))
>> +                       return ilb;
>> +       }
> 
> What will happen if cpu1 is still idle currently && housekeeping?

Then it will run the ilb.

In the customer scenario, cpu1 will not be in the housekeeping mask.

Thanks,
Nick

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