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

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Message-ID: <xm26sh5korxh.fsf@bsegall-linux.svl.corp.google.com>
Date:   Mon, 18 Jun 2018 11:44:42 -0700
From:   bsegall@...gle.com
To:     Xunlei Pang <xlpang@...ux.alibaba.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Ben Segall <bsegall@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched/fair: Fix bandwidth timer clock drift condition

Xunlei Pang <xlpang@...ux.alibaba.com> writes:

> 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 structure
> 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).

It might just be simplest to revert the comparison change - if we read a
torn value, the worst that happens is we extend incorrectly, and that
is exactly what happens if we just read the old value.

An extra int isn't exactly the worst thing though, so whichever.

>
> This fix is also needed by the following patch.
>
> Fixes: 51f2176d74ac ("sched/fair: Fix unlocked reads of some cfs_b->quota/period")
> Cc: Ben Segall <bsegall@...gle.com>
> Signed-off-by: Xunlei Pang <xlpang@...ux.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..9f384264e832 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;
> @@ -4629,6 +4631,7 @@ static int assign_cfs_rq_runtime(struct cfs_rq *cfs_rq)
>  		}
>  	}
>  	expires = cfs_b->runtime_expires;
> +	expires_seq = cfs_b->expires_seq;
>  	raw_spin_unlock(&cfs_b->lock);
>  
>  	cfs_rq->runtime_remaining += amount;
> @@ -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;
>  }
> @@ -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;