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

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Date:   Mon, 22 May 2017 13:57:36 -0600
From:   "Christ, Austin" <austinwc@...eaurora.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Jeffrey Hugo <jhugo@...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

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.
>> Another idea might be to check if the LBF_ALL_PINNED is set when we
>> check if we should clean the imbalance flag.
>>
>> @@ -8307,14 +8307,13 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>>           * We reach balance although we may have faced some affinity
>>           * constraints. Clear the imbalance flag if it was set.
>>           */
>> -       if (sd_parent) {
>> +       if (sd_parent && !(env.flags & LBF_ALL_PINNED)) {
>>                  int *group_imbalance = &sd_parent->groups->sgc->imbalance;
>>
>>                  if (*group_imbalance)
>>                          *group_imbalance = 0;
>>          }
> [...]

[1] - 
http://elixir.free-electrons.com/linux/latest/source/kernel/sched/fair.c#L8140

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