[SRU][B][PATCH 1/1] sched/fair: Fix bandwidth timer clock drift condition
Connor Kuehl
connor.kuehl at canonical.com
Fri Jun 28 14:25:44 UTC 2019
On 6/28/19 6:09 AM, Stefan Bader wrote:
> On 21.06.19 20:18, Connor Kuehl wrote:
>> 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.
>
> Would help if you supplied links to the patch or whatever bugzilla comment.
Sorry. Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=198197
Links to this patch on LKML: https://lkml.org/lkml/2019/5/17/581
>
>>
>>> ---
>>> 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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