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

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Date:   Thu, 18 May 2017 08:31:55 -0600
From:   Jeffrey Hugo <jhugo@...eaurora.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Austin Christ <austinwc@...eaurora.org>,
        Tyler Baicar <tbaicar@...eaurora.org>
Subject: Re: [RFC 1/2] sched/fair: Fix load_balance() affinity redo path

On 5/15/2017 8:56 AM, Dietmar Eggemann wrote:
> On 12/05/17 21:57, Jeffrey Hugo wrote:
>> On 5/12/2017 2:47 PM, Peter Zijlstra wrote:
>>> On Fri, May 12, 2017 at 11:01:37AM -0600, Jeffrey Hugo wrote:
>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>> index d711093..8f783ba 100644
>>>> --- a/kernel/sched/fair.c
>>>> +++ b/kernel/sched/fair.c
>>>> @@ -8219,8 +8219,19 @@ 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)) {
>>>> +            struct cpumask tmp;
>>>
>>> You cannot have cpumask's on stack.
>>
>> Well, we need a temp variable to store the intermediate values since the
>> cpumask_* operations are somewhat limited, and require a "storage"
>> parameter.
>>
>> Do you have any suggestions to meet all of these requirements?
> 
> What about we use env.dst_grpmask and check if cpus is an improper
> subset of env.dst_grpmask? In this case we have to get rid of
> setting env.dst_grpmask = NULL in case of CPU_NEWLY_IDLE which is
> IMHO not an issue since it's idle is passed via env into
> can_migrate_task().
> And cpus has to be and'ed with sched_domain_span(env.sd).
> 
> I'm not sure if this will work with 'not fully connected NUMA' (SD_OVERLAP)
> though ...

Hmm.  I follow the idea, but I'm not too confident in the SD_OVERLAP 
case, and looking at your proposed code, it seems invasive to me - 
changes are needed in what would otherwise be unrelated sections of 
code.  I'd prefer not to go in that direction.  Also, it appears that 
the dst_cpu is still considered as a source for load.

We've got a different idea to address the stack issue, and still keep 
the change "contained", which I'll roll into a V2 today or tomorrow.

-- 
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.