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

[bsegall@google.com]

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.


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