[SRU][B][PATCH 1/1] sched/fair: Fix bandwidth timer clock drift condition

[Connor Kuehl]

On 6/14/19 8:51 AM, Khalid Elmously wrote:
> From: Xunlei Pang <xlpang at linux.alibaba.com>
> 
> BugLink: https://bugs.launchpad.net/bugs/1832151
> 
> I noticed that cgroup task groups constantly get throttled even
> if they have low CPU usage, this causes some jitters on the response
> time to some of our business containers when enabling CPU quotas.
> 
> 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 steadily
> 
> 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 this commit:
> 
>   a9cf55b28610 ("sched: Expire invalid runtime")
> 
> ... but was changed to be the current implementation due to its locking bug.
> 
> 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
> uses them to figure out if clock drift happens (true if they are equal).
> 
> Signed-off-by: Xunlei Pang <xlpang at linux.alibaba.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org>
> Reviewed-by: Ben Segall <bsegall at google.com>
> Cc: Linus Torvalds <torvalds at linux-foundation.org>
> Cc: Peter Zijlstra <peterz at infradead.org>
> Cc: Thomas Gleixner <tglx at linutronix.de>
> Fixes: 51f2176d74ac ("sched/fair: Fix unlocked reads of some cfs_b->quota/period")
> Link: http://lkml.kernel.org/r/20180620101834.24455-1-xlpang@linux.alibaba.com
> Signed-off-by: Ingo Molnar <mingo at kernel.org>
> (backported from commit 512ac999d2755d2b7109e996a76b6fb8b888631d)
> [ kmously: Adjusted for different definitions of struct cfs_bandwidth and struct
>  cfs_rq ]
> Signed-off-by: Khalid Elmously <khalid.elmously at canonical.com>

This looks good to me, but the bugzilla links to another patch that's on
its way upstream that claims to follow up on a regression introduced by
this patch. Should that patch also be included here? I only ask because
I'm not sure I have all the information/knowledge to form an opinion on
that follow-up patch.

> ---
>  kernel/sched/fair.c  | 14 ++++++++------
>  kernel/sched/sched.h |  6 +++++-
>  2 files changed, 13 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 61365fcbe148..2ec80e0822a5 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4413,6 +4413,7 @@ 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)
> @@ -4435,6 +4436,7 @@ 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;
> @@ -4451,6 +4453,7 @@ 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);
>  
> @@ -4460,8 +4463,10 @@ 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;
>  }
> @@ -4487,12 +4492,9 @@ 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 41be9d48380f..3798f948477f 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -280,8 +280,11 @@ struct cfs_bandwidth {
>  	u64 quota, runtime;
>  	s64 hierarchical_quota;
>  	u64 runtime_expires;
> +	int expires_seq;
>  
> -	int idle, period_active;
> +
> +	short idle;
> +	short period_active;
>  	struct hrtimer period_timer, slack_timer;
>  	struct list_head throttled_cfs_rq;
>  
> @@ -490,6 +493,7 @@ struct cfs_rq {
>  
>  #ifdef CONFIG_CFS_BANDWIDTH
>  	int runtime_enabled;
> +	int expires_seq;
>  	u64 runtime_expires;
>  	s64 runtime_remaining;
>  
> 


-- 
Connor
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