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

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Date:   Fri, 4 May 2018 10:22:01 -0700
From:   Rohit Jain <rohit.k.jain@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     matt@...eblueprint.co.uk, mingo@...nel.org,
        dhaval.giani@...cle.com, subhra.mazumdar@...cle.com,
        steven.sistare@...cle.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC] sched/core: Don't schedule threads on pre-empted vcpus

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.