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SubjectRe: [Patch] sched/fair: Avoid throttle_list starvation with low cfs quota
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On 10.10.2018 21:37, Phil Auld wrote:
> On Wed, Oct 10, 2018 at 10:49:25AM -0700 bsegall@google.com wrote:
>> Ingo Molnar <mingo@kernel.org> writes:
>>
>>> I've Cc:-ed a handful of gents who worked on CFS bandwidth details to widen the discussion.
>>> Patch quoted below.
>>>
>>> Looks like a real bug that needs to be fixed - and at first sight the quota of 1000 looks very
>>> low - could we improve the arithmetics perhaps?
>>>
>>> A low quota of 1000 is used because there's many VMs or containers provisioned on the system
>>> that is triggering the bug, right?
>>>
>>> Thanks,
>>>
>>> Ingo
>>>
>>> * Phil Auld <pauld@redhat.com> wrote:
>>>
>>>> From: "Phil Auld" <pauld@redhat.com>
>>>>
>>>> sched/fair: Avoid throttle_list starvation with low cfs quota
>>>>
>>>> With a very low cpu.cfs_quota_us setting, such as the minimum of 1000,
>>>> distribute_cfs_runtime may not empty the throttled_list before it runs
>>>> out of runtime to distribute. In that case, due to the change from
>>>> c06f04c7048 to put throttled entries at the head of the list, later entries
>>>> on the list will starve. Essentially, the same X processes will get pulled
>>>> off the list, given CPU time and then, when expired, get put back on the
>>>> head of the list where distribute_cfs_runtime will give runtime to the same
>>>> set of processes leaving the rest.
>>>>
>>>> Fix the issue by setting a bit in struct cfs_bandwidth when
>>>> distribute_cfs_runtime is running, so that the code in throttle_cfs_rq can
>>>> decide to put the throttled entry on the tail or the head of the list. The
>>>> bit is set/cleared by the callers of distribute_cfs_runtime while they hold
>>>> cfs_bandwidth->lock.
>>>>
>>>> Signed-off-by: Phil Auld <pauld@redhat.com>
>>>> Fixes: c06f04c70489 ("sched: Fix potential near-infinite distribute_cfs_runtime() loop")
>>>> Cc: Peter Zijlstra <peterz@infradead.org>
>>>> Cc: Ingo Molnar <mingo@kernel.org>
>>>> Cc: stable@vger.kernel.org
>>
>> Reviewed-by: Ben Segall <bsegall@google.com>
>>
>>
>> In theory this does mean the unfairness could still happen if distribute is still
>> running, but while a tiny quota makes it more likely, the fact that
>> we're not getting through much of the list makes it not really a worry.
>> If you wanted to be even more careful there could be some generation
>> counter or something, but it doesn't seem necessary.
>>
>
> Thanks for the review. Yeah, I thought about a few other approaches, not explicitly
> what you suggested, but they all complicated things. This one seemed the closest to
> "obviously correct".

I've sent patch about same problem couple years ago:
https://lore.kernel.org/patchwork/patch/750523/

I think my approach is closer to FIFO and more fair.

>
>
> Cheers,
> Phil
>
>
>>
>>>> ---
>>>>
>>>> This is easy to reproduce with a handful of cpu consumers. I use crash on
>>>> the live system. In some cases you can simply look at the throttled list and
>>>> see the later entries are not changing:
>>>>
>>>> crash> list cfs_rq.throttled_list -H 0xffff90b54f6ade40 -s cfs_rq.runtime_remaining | paste - - | awk '{print $1" "$4}' | pr -t -n3
>>>> 1 ffff90b56cb2d200 -976050
>>>> 2 ffff90b56cb2cc00 -484925
>>>> 3 ffff90b56cb2bc00 -658814
>>>> 4 ffff90b56cb2ba00 -275365
>>>> 5 ffff90b166a45600 -135138
>>>> 6 ffff90b56cb2da00 -282505
>>>> 7 ffff90b56cb2e000 -148065
>>>> 8 ffff90b56cb2fa00 -872591
>>>> 9 ffff90b56cb2c000 -84687
>>>> 10 ffff90b56cb2f000 -87237
>>>> 11 ffff90b166a40a00 -164582
>>>> crash> list cfs_rq.throttled_list -H 0xffff90b54f6ade40 -s cfs_rq.runtime_remaining | paste - - | awk '{print $1" "$4}' | pr -t -n3
>>>> 1 ffff90b56cb2d200 -994147
>>>> 2 ffff90b56cb2cc00 -306051
>>>> 3 ffff90b56cb2bc00 -961321
>>>> 4 ffff90b56cb2ba00 -24490
>>>> 5 ffff90b166a45600 -135138
>>>> 6 ffff90b56cb2da00 -282505
>>>> 7 ffff90b56cb2e000 -148065
>>>> 8 ffff90b56cb2fa00 -872591
>>>> 9 ffff90b56cb2c000 -84687
>>>> 10 ffff90b56cb2f000 -87237
>>>> 11 ffff90b166a40a00 -164582
>>>>
>>>> Sometimes it is easier to see by finding a process getting starved and looking
>>>> at the sched_info:
>>>>
>>>> crash> task ffff8eb765994500 sched_info
>>>> PID: 7800 TASK: ffff8eb765994500 CPU: 16 COMMAND: "cputest"
>>>> sched_info = {
>>>> pcount = 8,
>>>> run_delay = 697094208,
>>>> last_arrival = 240260125039,
>>>> last_queued = 240260327513
>>>> },
>>>> crash> task ffff8eb765994500 sched_info
>>>> PID: 7800 TASK: ffff8eb765994500 CPU: 16 COMMAND: "cputest"
>>>> sched_info = {
>>>> pcount = 8,
>>>> run_delay = 697094208,
>>>> last_arrival = 240260125039,
>>>> last_queued = 240260327513
>>>> },
>>>>
>>>>
>>>> fair.c | 22 +++++++++++++++++++---
>>>> sched.h | 2 ++
>>>> 2 files changed, 21 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>> index 7fc4a371bdd2..f88e00705b55 100644
>>>> --- a/kernel/sched/fair.c
>>>> +++ b/kernel/sched/fair.c
>>>> @@ -4476,9 +4476,13 @@ static void throttle_cfs_rq(struct cfs_rq *cfs_rq)
>>>>
>>>> /*
>>>> * Add to the _head_ of the list, so that an already-started
>>>> - * distribute_cfs_runtime will not see us
>>>> + * distribute_cfs_runtime will not see us. If disribute_cfs_runtime is
>>>> + * not running add to the tail so that later runqueues don't get starved.
>>>> */
>>>> - list_add_rcu(&cfs_rq->throttled_list, &cfs_b->throttled_cfs_rq);
>>>> + if (cfs_b->distribute_running)
>>>> + list_add_rcu(&cfs_rq->throttled_list, &cfs_b->throttled_cfs_rq);
>>>> + else
>>>> + list_add_tail_rcu(&cfs_rq->throttled_list, &cfs_b->throttled_cfs_rq);
>>>>
>>>> /*
>>>> * If we're the first throttled task, make sure the bandwidth
>>>> @@ -4622,14 +4626,16 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
>>>> * in us over-using our runtime if it is all used during this loop, but
>>>> * only by limited amounts in that extreme case.
>>>> */
>>>> - while (throttled && cfs_b->runtime > 0) {
>>>> + while (throttled && cfs_b->runtime > 0 && !cfs_b->distribute_running) {
>>>> runtime = cfs_b->runtime;
>>>> + cfs_b->distribute_running = 1;
>>>> raw_spin_unlock(&cfs_b->lock);
>>>> /* we can't nest cfs_b->lock while distributing bandwidth */
>>>> runtime = distribute_cfs_runtime(cfs_b, runtime,
>>>> runtime_expires);
>>>> raw_spin_lock(&cfs_b->lock);
>>>>
>>>> + cfs_b->distribute_running = 0;
>>>> throttled = !list_empty(&cfs_b->throttled_cfs_rq);
>>>>
>>>> cfs_b->runtime -= min(runtime, cfs_b->runtime);
>>>> @@ -4740,6 +4746,11 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
>>>>
>>>> /* confirm we're still not at a refresh boundary */
>>>> raw_spin_lock(&cfs_b->lock);
>>>> + if (cfs_b->distribute_running) {
>>>> + raw_spin_unlock(&cfs_b->lock);
>>>> + return;
>>>> + }
>>>> +
>>>> if (runtime_refresh_within(cfs_b, min_bandwidth_expiration)) {
>>>> raw_spin_unlock(&cfs_b->lock);
>>>> return;
>>>> @@ -4749,6 +4760,9 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
>>>> runtime = cfs_b->runtime;
>>>>
>>>> expires = cfs_b->runtime_expires;
>>>> + if (runtime)
>>>> + cfs_b->distribute_running = 1;
>>>> +
>>>> raw_spin_unlock(&cfs_b->lock);
>>>>
>>>> if (!runtime)
>>>> @@ -4759,6 +4773,7 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
>>>> raw_spin_lock(&cfs_b->lock);
>>>> if (expires == cfs_b->runtime_expires)
>>>> cfs_b->runtime -= min(runtime, cfs_b->runtime);
>>>> + cfs_b->distribute_running = 0;
>>>> raw_spin_unlock(&cfs_b->lock);
>>>> }
>>>>
>>>> @@ -4867,6 +4882,7 @@ void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
>>>> cfs_b->period_timer.function = sched_cfs_period_timer;
>>>> hrtimer_init(&cfs_b->slack_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
>>>> cfs_b->slack_timer.function = sched_cfs_slack_timer;
>>>> + cfs_b->distribute_running = 0;
>>>> }
>>>>
>>>> static void init_cfs_rq_runtime(struct cfs_rq *cfs_rq)
>>>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>>>> index 455fa330de04..9683f458aec7 100644
>>>> --- a/kernel/sched/sched.h
>>>> +++ b/kernel/sched/sched.h
>>>> @@ -346,6 +346,8 @@ struct cfs_bandwidth {
>>>> int nr_periods;
>>>> int nr_throttled;
>>>> u64 throttled_time;
>>>> +
>>>> + bool distribute_running;
>>>> #endif
>>>> };
>>>>
>>>>
>>>>
>>>> --
>

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