Re: [PATCH v2 1/2] sched/fair: Fix bandwidth timer clock drift condition

[bsegall@google.com]

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>

> 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;