linux-kernel - Re: [PATCH V3 1/2] sched/fair: Fix load

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Date:   Tue, 23 May 2017 16:05:10 -0600
From:   Jeffrey Hugo <jhugo@...eaurora.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>,
        "Christ, Austin" <austinwc@...eaurora.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Cc:     Tyler Baicar <tbaicar@...eaurora.org>,
        Timur Tabi <timur@...eaurora.org>
Subject: Re: [PATCH V3 1/2] sched/fair: Fix load_balance() affinity redo path

On 5/23/2017 5:45 AM, Dietmar Eggemann wrote:
> Hey Austin,
> 
> On 22/05/17 20:57, Christ, Austin wrote:
>> Hey Dietmar,
>>
>>
>> On 5/22/2017 3:48 AM, Dietmar Eggemann wrote:
>>> On 19/05/17 14:31, Dietmar Eggemann wrote:
>>>> On 18/05/17 20:36, Jeffrey Hugo wrote:
>>>>
>>>> [...]
>>>>
>>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>>> index d711093..a5d41b1 100644
>>>>> --- a/kernel/sched/fair.c
>>>>> +++ b/kernel/sched/fair.c
>>>>> @@ -8220,7 +8220,24 @@ static int load_balance(int this_cpu, struct
>>>>> rq *this_rq,
>>>>>            /* All tasks on this runqueue were pinned by CPU affinity */
>>>>>            if (unlikely(env.flags & LBF_ALL_PINNED)) {
>>>>>                cpumask_clear_cpu(cpu_of(busiest), cpus);
>>>>> -            if (!cpumask_empty(cpus)) {
>>>>> +            /*
>>>>> +             * dst_cpu is not a valid busiest cpu in the following
>>>>> +             * check since load cannot be pulled from dst_cpu to be
>>>>> +             * put on dst_cpu.
>>>>> +             */
>>>>> +            cpumask_clear_cpu(env.dst_cpu, cpus);
>>>>> +            /*
>>>>> +             * Go back to "redo" iff the load-balance cpumask
>>>>> +             * contains other potential busiest cpus for the
>>>>> +             * current sched domain.
>>>>> +             */
>>>>> +            if (cpumask_intersects(cpus, sched_domain_span(env.sd))) {
>>>>> +                /*
>>>>> +                 * Now that the check has passed, reenable
>>>>> +                 * dst_cpu so that load can be calculated on
>>>>> +                 * it in the redo path.
>>>>> +                 */
>>>>> +                cpumask_set_cpu(env.dst_cpu, cpus);
>>>> IMHO, this will work nicely and its way easier.
>>> This was too quick ... if we still have other potential dst cpus
>>> available and cpu_of(busiest) is the latest src cpu then this will fail.
>>>
>>> It does work on sd with 'group_weight == 1', e.g. your MC sd 'sd->child
>>> == NULL'.
>>>
>>> But IMHO 'group_imbalance' propagation has to work on higher sd levels
>>> as well.
>> Can you clarify the fail case you are seeing? We are only aware of
>> dst_cpu being changed under [1] where a dst_cpu will try to move work to
>> one of its sched_group siblings.
>>
>> I'm also not entirely sure I understand what you mean about the flag
>> being propagated to higher sd levels.
> 
> The propagation of 'imbalance' information should not only happen
> between lowest sd (sd->child == NULL) and its parent (MC->DIE in your
> example) but between all {sd, sd->parent} pairs.
> 
> Imagine your machine had another sd on top of DIE.
> 
> I recreated the issue I pointed out on my hikey board (2*4) (w/o this
> extra sd on top of DIE), hotplug-ed out cpu 2,3,6,7 so I have a system
> with the following DIE sched_groups (sg):
> 
> sg1(0,1) and sg2(4,5) <- the DIE level sg's contain more than 1 logical cpu.
> 
> As a workload I run 4 25% tasks affine to [0,1]. These tasks are
> 'SOURCE' PINNED for a DIE lb sg2<-sg1.
> 
> With:
> 
> if (unlikely(env.flags & LBF_ALL_PINNED)) {
>    cpumask_clear_cpu(cpu_of(busiest), cpus);
>      if (!cpumask_empty(cpus)) {
>        ...
> 
>        printk("goto redo: sd=%s dst_cpu=%d src_cpu=%d cpus=%*pbl
>                dst_grpmask=%*pbl\n",
>                sd->name, env.dst_cpu, cpu_of(busiest),
>                cpumask_pr_args(cpus),
>                cpumask_pr_args(env.dst_grpmask));
> 
>        goto redo;
>      }
> 
> While running the workload I sometimes get:
> 
> ...
> goto redo: sd=DIE dst_cpu=4 src_cpu=1 cpus=0,4-5 dst_grpmask=4-5
> goto redo: sd=DIE dst_cpu=4 src_cpu=0 cpus=4-5 dst_grpmask=4-5
> ...
> 
> So even though 'redo' handling has tried both possible src_cpu's we
> would still enter another 'redo' path even you remove dst_cpu=4
> temporarily because of cpu=5.
> 
> You could replace:
> 
> cpumask_clear_cpu(env.dst_cpu, cpus)
> cpumask_set_cpu(env.dst_cpu, cpus)
> 
> with
> 
> cpumask_andnot(cpus, cpus, env.dst_grpmask)
> cpumask_or(cpus, cpus, env.dst_grpmask)
> 
> but then env.dst_grpmask can't be set to NULL for CPU_NEWLY_IDLE and
> you're almost at the snippet I sent out for v1:
> https://marc.info/?l=linux-kernel&m=149486020010389&w=2
> 
> [...]
> 

I see what you mean.  You are right, our current proposal is flawed in 
this regard.  We'll take a look for a bit and see if we come up with any 
other ideas.

-- 
Jeffrey Hugo
Qualcomm Datacenter Technologies as an affiliate of Qualcomm 
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.