From: Nicholas Piggin <npiggin@gmail.com>
To: Wanpeng Li <kernellwp@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
LKML <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] kernel/sched: run nohz idle load balancer on HK_FLAG_MISC CPUs
Date: Sun, 28 Apr 2019 21:58:34 +1000 [thread overview]
Message-ID: <1556452479.a2f15ax2p1.astroid@bobo.none> (raw)
In-Reply-To: <CANRm+CzLmRHtpAawBSknXyznw25ym_otnYJYYxAho-E46K3BZg@mail.gmail.com>
Wanpeng Li's on April 28, 2019 5:01 pm:
> On Fri, 12 Apr 2019 at 12:27, Nicholas Piggin <npiggin@gmail.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@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;
>> + }
>
> 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
next prev parent reply other threads:[~2019-04-28 11:58 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-12 4:26 [PATCH] kernel/sched: run nohz idle load balancer on HK_FLAG_MISC CPUs Nicholas Piggin
2019-04-25 11:56 ` Peter Zijlstra
2019-04-26 4:40 ` Nicholas Piggin
2019-04-28 7:01 ` Wanpeng Li
2019-04-28 11:58 ` Nicholas Piggin [this message]
2019-04-29 6:38 ` [tip:sched/core] sched/nohz: Run NOHZ " tip-bot for Nicholas Piggin
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