linux-kernel - Re: [RFC] sched/core: Don't schedule threads on pre-empted vcpus

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Date:   Fri, 4 May 2018 10:37:48 -0700
From:   Rohit Jain <rohit.k.jain@...cle.com>
To:     Steven Sistare <steven.sistare@...cle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, matt@...eblueprint.co.uk,
        mingo@...nel.org, dhaval.giani@...cle.com,
        subhra.mazumdar@...cle.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC] sched/core: Don't schedule threads on pre-empted vcpus

Hi Steve,


On 05/04/2018 10:32 AM, Steven Sistare wrote:
> On 5/4/2018 1:22 PM, Rohit Jain wrote:
>> Hi Peter,
>>
>> On 05/04/2018 02:47 AM, Peter Zijlstra wrote:
>>> On Wed, May 02, 2018 at 01:52:10PM -0700, Rohit Jain wrote:
>>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>>>> index 5e10aae..75d1ecf 100644
>>>> --- a/kernel/sched/core.c
>>>> +++ b/kernel/sched/core.c
>>>> @@ -4033,6 +4033,9 @@ int idle_cpu(int cpu)
>>>>            return 0;
>>>>    #endif
>>>>    +    if (vcpu_is_preempted(cpu))
>>>> +        return 0;
>>>> +
>>>>        return 1;
>>>>    }
>>> Basically OK with this, but did you consider idle_cpu() usage outside of
>>> select_idle_sibling()?
>>>
>>> For instance, I think got_nohz_idle_kick() isn't quite right with this
>>> on. Similarly for scheduler_tick(), that wants the actual idle state.
>> As far as intent is concerned, yes I agree you might be right. I left
>> the VM running for a couple of days, didn't see anything weird however.
>>
>> We could add a check at each of those places or something to that effect
>> if this is an issue. Please let me know how you want to proceed.
> The point is that some idle_cpu() call sites should consider preemption state
> and some should not, and they must be considered on a case by case basis.  You
> could define a new accessor to abstract the difference, and call it from
> select_idle_sibling and anywhere else it makes sense.
>
> available_idle_cpu()
> {
>    return idle_cpu() && !vcpu_is_preempted()
> }

Great! That's what I was thinking as "something to that effect" :)

Thanks,
Rohit