linux-kernel - Re: [PATCH v3 1/7] sched/fair: Ignore percpu threads for imbalance pulls

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Date:   Tue, 16 Mar 2021 21:33:26 +0530
From:   Chandra Sekhar Lingutla <clingutla@...eaurora.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qais Yousef <qais.yousef@....com>,
        Quentin Perret <qperret@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v3 1/7] sched/fair: Ignore percpu threads for imbalance
 pulls

Hi Dietmar,

On 3/16/2021 9:19 PM, Dietmar Eggemann wrote:
> On 11/03/2021 13:05, Valentin Schneider wrote:
>> From: Lingutla Chandrasekhar <clingutla@...eaurora.org>
>>
>> In load balancing, when balancing group is unable to pull task
>> due to ->cpus_ptr constraints from busy group, then it sets
>> LBF_SOME_PINNED to lb env flags, as a consequence, sgc->imbalance
>> is set for its parent domain level. which makes the group
>> classified as imbalance to get help from another balancing cpu.
>>
>> Consider a 4-CPU big.LITTLE system with CPUs 0-1 as LITTLEs and
> Does it have to be a big.LITTLE system? I assume this issue also happens
> on an SMP system.

Yah, issue can happen on SMP system as well.  I will let Valentin update 
the commit text on
his next version of this series.

>> CPUs 2-3 as Bigs with below scenario:
>> - CPU0 doing newly_idle balancing
>> - CPU1 running percpu kworker and RT task (small tasks)
> What's the role of the small RT task here in the story?
This is to satisfy 'busiest->nr_running > 1' checks.
>> - CPU2 running 2 big tasks
>> - CPU3 running 1 medium task
>>
>> While CPU0 is doing newly_idle load balance at MC level, it fails to
>> pull percpu kworker from CPU1 and sets LBF_SOME_PINNED to lb env flag
>> and set sgc->imbalance at DIE level domain. As LBF_ALL_PINNED not cleared,
>> it tries to redo the balancing by clearing CPU1 in env cpus, but it don't
>> find other busiest_group, so CPU0 stops balacing at MC level without
>> clearing 'sgc->imbalance' and restart the load balacing at DIE level.
>>
>> And CPU0 (balancing cpu) finds LITTLE's group as busiest_group with group
>> type as imbalance, and Bigs that classified the level below imbalance type
>> would be ignored to pick as busiest, and the balancing would be aborted
>> without pulling any tasks (by the time, CPU1 might not have running tasks).
>>
>> It is suboptimal decision to classify the group as imbalance due to
>> percpu threads. So don't use LBF_SOME_PINNED for per cpu threads.
> This sentence mentioned per-cpu threads (and so does the patch name) but
> the implementation (only) deals with per-cpu kernel threads. IMHO, it
> would be good to align this.
I will let Valentin update this on next version.
>> Signed-off-by: Lingutla Chandrasekhar <clingutla@...eaurora.org>
>> [Use kthread_is_per_cpu() rather than p->nr_cpus_allowed]
>> Signed-off-by: Valentin Schneider <valentin.schneider@....com>
>> ---
>>   kernel/sched/fair.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 2e2ab1e00ef9..83aea97fbf22 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -7565,6 +7565,10 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>>   	if (throttled_lb_pair(task_group(p), env->src_cpu, env->dst_cpu))
>>   		return 0;
>>   
>> +	/* Disregard pcpu kthreads; they are where they need to be. */
>> +	if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p))
>> +		return 0;
>> +
>>   	if (!cpumask_test_cpu(env->dst_cpu, p->cpus_ptr)) {
>>   		int cpu;
>>   
>>