Re: your mail

Nicholas Piggin
2019-04-12T03:23:47Z

Peter Zijlstra's on April 11, 2019 8:53 pm:
> Was this supposed to be patch 6/5 of your previous series?

Dang, I screwed up the headers? Thanks for the ping, I will resend.

It is standalone. It seems more suited to the scheduler tree than the
timers one, but your call.

It is generally of more use when CPU0 is _not_ a housekeeping one,
and that's where I've done most testing, but I don't see any hard
dependency.

Thanks,
Nick

>
> On Thu, Apr 11, 2019 at 04:05:36PM +1000, Nicholas Piggin wrote:
>> Date: Tue, 9 Apr 2019 20:23:16 +1000
>> Subject: [PATCH] kernel/sched: run nohz idle load balancer on HK_FLAG_MISC
>> CPUs
>>
>> 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@gmail.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;
>> + }
>>
>> return nr_cpu_ids;
>> }
>>
>> /*
>> - * Kick a CPU to do the nohz balancing, if it is time for it. We pick the
>> - * nohz_load_balancer CPU (if there is one) otherwise fallback to any idle
>> - * CPU (if there is one).
>> + * Kick a CPU to do the nohz balancing, if it is time for it. We pick any
>> + * idle CPU in the HK_FLAG_MISC housekeeping set (if there is one).
>> */
>> static void kick_ilb(unsigned int flags)
>> {
>> --
>> 2.20.1
>>
>