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SubjectRe: [PATCH 2/2] sched/fair: Advance global expiration when period timer is restarted
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On 6/19/18 2:58 AM, bsegall@google.com wrote:
> Xunlei Pang <xlpang@linux.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@linux.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

>
>> + cfs_b->expires_seq++;
>> + hrtimer_start_expires(&cfs_b->period_timer, HRTIMER_MODE_ABS_PINNED);
>> }
>>
>> static void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b)

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