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Subject: Re: [PATCH] sched/fair: don't assign runtime for throttled cfs_rq
To:     bsegall@google.com, Valentin Schneider <valentin.schneider@arm.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, shanpeic@linux.alibaba.com,
        xlpang@linux.alibaba.com
References: <20190814180021.165389-1-liangyan.peng@linux.alibaba.com>
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From:   Liangyan <liangyan.peng@linux.alibaba.com>
Message-ID: <5cfadb62-10b7-d16b-7f30-f3573bb04844@linux.alibaba.com>
Date:   Tue, 27 Aug 2019 10:45:00 +0800
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On 19/8/27 上午1:38, bsegall@google.com wrote:
> Valentin Schneider <valentin.schneider@arm.com> writes:
> 
>> On 23/08/2019 21:00, bsegall@google.com wrote:
>> [...]
>>> Could you mention in the message that this a throttled cfs_rq can have
>>> account_cfs_rq_runtime called on it because it is throttled before
>>> idle_balance, and the idle_balance calls update_rq_clock to add time
>>> that is accounted to the task.
>>>
>>
>> Mayhaps even a comment for the extra condition.
>>
>>> I think this solution is less risky than unthrottling
>>> in this area, so other than that:
>>>
>>> Reviewed-by: Ben Segall <bsegall@google.com>
>>>
>>
>> If you don't mind squashing this in:
>>
>> -----8<-----
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index b1d9cec9b1ed..b47b0bcf56bc 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -4630,6 +4630,10 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b, u64 remaining)
>>   		if (!cfs_rq_throttled(cfs_rq))
>>   			goto next;
>>   
>> +		/* By the above check, this should never be true */
>> +		WARN_ON(cfs_rq->runtime_remaining > 0);
>> +
>> +		/* Pick the minimum amount to return to a positive quota state */
>>   		runtime = -cfs_rq->runtime_remaining + 1;
>>   		if (runtime > remaining)
>>   			runtime = remaining;
>> ----->8-----
>>
>> I'm not adamant about the extra comment, but the WARN_ON would be nice IMO.
>>
>>
>> @Ben, do you reckon we want to strap
>>
>> Cc: <stable@vger.kernel.org>
>> Fixes: ec12cb7f31e2 ("sched: Accumulate per-cfs_rq cpu usage and charge against bandwidth")
>>
>> to the thing? AFAICT the pick_next_task_fair() + idle_balance() dance you
>> described should still be possible on that commit.
> 
> I'm not sure about stable policy in general, but it seems reasonable.
> The WARN_ON might want to be WARN_ON_ONCE, and it seems fine to have it
> or not.

Thanks Ben and Valentin for all of the comments. Per Xunlei's 
suggestion, I used SCHED_WARN_ON instead in v3. Regarding whether cc 
stable, I'm also not sure.

> 
>>
>>
>> Other than that,
>>
>> Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
>>
>> [...]