Date:   Wed, 20 Jun 2018 18:26:10 +0800
From:   Xunlei Pang <xlpang@...ux.alibaba.com>
To:     bsegall@...gle.com
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] sched/fair: Advance global expiration when period
 timer is restarted

On 6/20/18 2:06 PM, Xunlei Pang wrote:
> On 6/20/18 1:49 AM, bsegall@...gle.com wrote:
>> Xunlei Pang <xlpang@...ux.alibaba.com> writes:
>>
>>> On 6/19/18 2:58 AM, bsegall@...gle.com wrote:
>>>> Xunlei Pang <xlpang@...ux.alibaba.com> writes:
>>>>
>>>>> I noticed the group frequently got throttled even it consumed
>>>>> low cpu usage, this caused some jitters on the response time
>>>>> to some of our business containers enabling cpu quota.
>>>>>
>>>>> It's very easy to reproduce:
>>>>> mkdir /sys/fs/cgroup/cpu/test
>>>>> cd /sys/fs/cgroup/cpu/test
>>>>> echo 100000 > cpu.cfs_quota_us
>>>>> echo $$ > tasks
>>>>> then repeat:
>>>>> cat cpu.stat |grep nr_throttled  // nr_throttled will increase
>>>>>
>>>>> After some analysis, we found that cfs_rq::runtime_remaining will
>>>>> be cleared by expire_cfs_rq_runtime() due to two equal but stale
>>>>> "cfs_{b|q}->runtime_expires" after period timer is re-armed.
>>>>
>>>> If this is after the first patch, then this is no longer what should
>>>> happen, and instead it would incorrectly /keep/ old local cfs_rq
>>>> runtime, and not __refill global runtime until the period.
>>>
>>> Exactly, I can improve the changelog.
>>>
>>>>
>>>>
>>>>>
>>>>> The global expiration should be advanced accordingly when the
>>>>> bandwidth period timer is restarted.
>>>>>
>>>>> Signed-off-by: Xunlei Pang <xlpang@...ux.alibaba.com>
>>>>> ---
>>>>>  kernel/sched/fair.c | 15 ++++++++++-----
>>>>>  1 file changed, 10 insertions(+), 5 deletions(-)
>>>>>
>>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>>> index 9f384264e832..bb006e671609 100644
>>>>> --- a/kernel/sched/fair.c
>>>>> +++ b/kernel/sched/fair.c
>>>>> @@ -5204,13 +5204,18 @@ static void init_cfs_rq_runtime(struct cfs_rq *cfs_rq)
>>>>>  
>>>>>  void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
>>>>>  {
>>>>> +	u64 overrun;
>>>>> +
>>>>>  	lockdep_assert_held(&cfs_b->lock);
>>>>>  
>>>>> -	if (!cfs_b->period_active) {
>>>>> -		cfs_b->period_active = 1;
>>>>> -		hrtimer_forward_now(&cfs_b->period_timer, cfs_b->period);
>>>>> -		hrtimer_start_expires(&cfs_b->period_timer, HRTIMER_MODE_ABS_PINNED);
>>>>> -	}
>>>>> +	if (cfs_b->period_active)
>>
>>>>> +		return;
>>>>> +
>>>>> +	cfs_b->period_active = 1;
>>>>> +	overrun = hrtimer_forward_now(&cfs_b->period_timer, cfs_b->period);
>>>>> +	cfs_b->runtime_expires += (overrun + 1) * ktime_to_ns(cfs_b->period);
>>>>
>>>> I think we actually want if (overrun) __refill_cfs_bandwidth_runtime(),
>>>> much like tg_set_cfs_bandwidth
>>>
>>> It's a little different, e.g. periods like below:
>>>
>>>            Pm
>>>            |
>>>            v
>>>   |-----|-----|
>>> Pn-1   Pn    Pn+1
>>>
>>> Assuming we start_cfs_bandwidth() at some moment Pm between "Pn" and
>>> "Pn+1", cfs_b->runtime_expires we want here should be "Pn+1", while
>>> __refill_cfs_bandwidth_runtime() forces it to be "Pm+period"(hrtimer
>>> expires at "Pn+1").
>>>
>>> Regards,
>>> Xunlei
>>>
>>
>> Hmm, yeah, it would wind up out of sync with the hrtimer unless we
>> restarted the period boundaries.
>>
>> Now that I look at it even more closely, I don't see how you're getting
>> problems like that, besides wasting tiny amounts of cpu time in expire:
>> we only disable the period timer when there has been a full period with
>> no activity, so we should have a full cfs_b->runtime, and a
>> runtime_expires from the period that was spent entirely idle (so it
>> should always be in the past). But with a working extend-local-deadline
>> check like after patch 1 it shouldn't /matter/ what the actual value of
>> runtime_expires is, we'll just waste tiny amounts of cpu time
>> incrementing runtime_expires every account_cfs_rq_runtime call rather
>> than aborting early.
>>
>> Actually, I think arguably the real issue may be in patch 1:
> 
> Ben,
> 
> Thanks for the comments.
> 
> Yes, the problematic throttle issue is solved by patch 1, and it's a
> different issue in patch 2. It wants to address another stale runtime
> issue(I will update the changelog in v2), specifically:
> An ever-running cfs_rq tends to retain some runtime(min_cfs_rq_runtime),
> it could be even more(sysctl_sched_cfs_bandwidth_slice) in the worst
> case, these runtime should be discarded once the period is advanced.
> 
> Without this patch expire_cfs_rq_runtime() will see the two equal
> cfs_rq{b}->expires_seq after period gets restarted after some idle,
> so it will fall into the "clock drift" branch and go on consuming
> the old runtime if any.
> 
> With the expiration update done in patch 2, we can ensure the stale
> cfs_rq->runtime_remaining gets cleared at expire_cfs_rq_runtime()
> once a new period begins.

I did some tests, as you said the throttle problem gets resovled by
patch 1. And stale cfs_rq->runtime_remaining can also be cleared
properly due to different expires_seq, now patch 2 is mainly to avoid
needless expire_cfs_rq_runtime() execution due to old expiration time.

I've sent v2, please have a look, thanks!

-Xunlei

> 
>>
>>>> @@ -4637,8 +4640,10 @@ static int assign_cfs_rq_runtime(struct cfs_rq *cfs_rq)
>>>>  	 * spread between our sched_clock and the one on which runtime was
>>>>  	 * issued.
>>>>  	 */
>>>> -	if ((s64)(expires - cfs_rq->runtime_expires) > 0)
>>>> +	if ((s64)(expires - cfs_rq->runtime_expires) > 0) {
>>>>  		cfs_rq->runtime_expires = expires;
>>>> +		cfs_rq->expires_seq = expires_seq;
>>>> +	}
>>>>  
>>>>  	return cfs_rq->runtime_remaining > 0;
>>>>  }
>>
>>
>> We don't want to clobber a tick-extended runtime_expires with the
>> original one, but we /do/ want update if expires_seq changes, no matter
>> what (rather than skipping updating both if we've tick-extended past the
>> /next/ period's runtime_expires).
>>
>> I think this should actually be
>>
>> if (cfs_rq->expires_seq != expires_seq) {
>> 	cfs_rq->runtime_expires = expires;
>> 	cfs_rq->expires_seq = expires_seq;
>> }
>>
>> Can you easily check and see if this fixes the issue without this second
>> patch? That would clear up how the actual problem is happening, and I
>> think it's a change we'd want anyways.
> 
> Agree, I will have it in v2.
> 
>>
>> That said, we should also just update runtime_expires like this patch
>> does regardless, and as mentioned we should have a full cfs_b->runtime
>> so we don't need a __refill like I was originally thinking.
>>
> Regards,
> Xunlei
> 

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