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

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Date:   Fri, 4 May 2018 13:32:09 -0400
From:   Steven Sistare <steven.sistare@...cle.com>
To:     Rohit Jain <rohit.k.jain@...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

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()
}

- Steve