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SubjectRe: [PATCH v2 1/2] sched/fair: Fix bandwidth timer clock drift condition
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On 6/21/18 1:01 AM, bsegall@google.com wrote:
> Xunlei Pang <xlpang@linux.alibaba.com> writes:
>
>> I noticed the group constantly 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 simple 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.
>>
>> The current condition to judge clock drift in expire_cfs_rq_runtime()
>> is wrong, the two runtime_expires are actually the same when clock
>> drift happens, so this condtion can never hit. The orginal design was
>> correctly done by commit a9cf55b28610 ("sched: Expire invalid runtime"),
>> but was changed to be the current one due to its locking issue.
>>
>> This patch introduces another way, it adds a new field in both structures
>> cfs_rq and cfs_bandwidth to record the expiration update sequence, and
>> use them to figure out if clock drift happens(true if they equal).
>>
>> Fixes: 51f2176d74ac ("sched/fair: Fix unlocked reads of some cfs_b->quota/period")
>> Cc: Ben Segall <bsegall@google.com>
>
> Reviewed-By: Ben Segall <bsegall@google.com>

Thanks Ben :-)

Hi Peter, could you please have a look at them?

Cc stable@vger.kernel.org

>
>> Signed-off-by: Xunlei Pang <xlpang@linux.alibaba.com>
>> ---
>> kernel/sched/fair.c | 14 ++++++++------
>> kernel/sched/sched.h | 6 ++++--
>> 2 files changed, 12 insertions(+), 8 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index e497c05aab7f..e6bb68d52962 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -4590,6 +4590,7 @@ void __refill_cfs_bandwidth_runtime(struct cfs_bandwidth *cfs_b)
>> now = sched_clock_cpu(smp_processor_id());
>> cfs_b->runtime = cfs_b->quota;
>> cfs_b->runtime_expires = now + ktime_to_ns(cfs_b->period);
>> + cfs_b->expires_seq++;
>> }
>>
>> static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
>> @@ -4612,6 +4613,7 @@ static int assign_cfs_rq_runtime(struct cfs_rq *cfs_rq)
>> struct task_group *tg = cfs_rq->tg;
>> struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(tg);
>> u64 amount = 0, min_amount, expires;
>> + int expires_seq;
>>
>> /* note: this is a positive sum as runtime_remaining <= 0 */
>> min_amount = sched_cfs_bandwidth_slice() - cfs_rq->runtime_remaining;
>> @@ -4628,6 +4630,7 @@ static int assign_cfs_rq_runtime(struct cfs_rq *cfs_rq)
>> cfs_b->idle = 0;
>> }
>> }
>> + expires_seq = cfs_b->expires_seq;
>> expires = cfs_b->runtime_expires;
>> raw_spin_unlock(&cfs_b->lock);
>>
>> @@ -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 (cfs_rq->expires_seq != expires_seq) {
>> + cfs_rq->expires_seq = expires_seq;
>> cfs_rq->runtime_expires = expires;
>> + }
>>
>> return cfs_rq->runtime_remaining > 0;
>> }
>> @@ -4664,12 +4669,9 @@ static void expire_cfs_rq_runtime(struct cfs_rq *cfs_rq)
>> * has not truly expired.
>> *
>> * Fortunately we can check determine whether this the case by checking
>> - * whether the global deadline has advanced. It is valid to compare
>> - * cfs_b->runtime_expires without any locks since we only care about
>> - * exact equality, so a partial write will still work.
>> + * whether the global deadline(cfs_b->expires_seq) has advanced.
>> */
>> -
>> - if (cfs_rq->runtime_expires != cfs_b->runtime_expires) {
>> + if (cfs_rq->expires_seq == cfs_b->expires_seq) {
>> /* extend local deadline, drift is bounded above by 2 ticks */
>> cfs_rq->runtime_expires += TICK_NSEC;
>> } else {
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index 6601baf2361c..e977e04f8daf 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -334,9 +334,10 @@ struct cfs_bandwidth {
>> u64 runtime;
>> s64 hierarchical_quota;
>> u64 runtime_expires;
>> + int expires_seq;
>>
>> - int idle;
>> - int period_active;
>> + short idle;
>> + short period_active;
>> struct hrtimer period_timer;
>> struct hrtimer slack_timer;
>> struct list_head throttled_cfs_rq;
>> @@ -551,6 +552,7 @@ struct cfs_rq {
>>
>> #ifdef CONFIG_CFS_BANDWIDTH
>> int runtime_enabled;
>> + int expires_seq;
>> u64 runtime_expires;
>> s64 runtime_remaining;

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