Re: [Patch] sched/fair: Avoid throttle_list starvation with low cfs quota

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

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
>>>>   };
>>>>   
>>>>
>>>>
>>>> -- 
>