linux-kernel - Re: your mail

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Message-Id: <1555039052.5o9b9e6rcr.astroid@bobo.none>
Date:   Fri, 12 Apr 2019 13:23:38 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Frederic Weisbecker <fweisbec@...il.com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: your mail

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@...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;
>> +	}
>>  
>>  	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
>> 
>