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;