Re: [PATCH v2 1/3] sched/fair: Disable runtime_enabled on dying rq

[Kirill Tkhai <ktkhai@parallels.com>]

В Ср, 25/06/2014 в 11:53 +0400, Kirill Tkhai пишет:
> В Вт, 24/06/2014 в 23:26 +0400, Kirill Tkhai пишет:
> > On 24.06.2014 23:13, bsegall@google.com wrote:
> > > Kirill Tkhai <tkhai@yandex.ru> writes:
> > > 
> > >> On 24.06.2014 21:03, bsegall@google.com wrote:
> > >>> Kirill Tkhai <ktkhai@parallels.com> writes:
> > >>>
> > >>>> We kill rq->rd on the CPU_DOWN_PREPARE stage:
> > >>>>
> > >>>> 	cpuset_cpu_inactive -> cpuset_update_active_cpus -> partition_sched_domains ->
> > >>>> 	-> cpu_attach_domain -> rq_attach_root -> set_rq_offline
> > >>>>
> > >>>> This unthrottles all throttled cfs_rqs.
> > >>>>
> > >>>> But the cpu is still able to call schedule() till
> > >>>>
> > >>>> 	take_cpu_down->__cpu_disable()
> > >>>>
> > >>>> is called from stop_machine.
> > >>>>
> > >>>> This case the tasks from just unthrottled cfs_rqs are pickable
> > >>>> in a standard scheduler way, and they are picked by dying cpu.
> > >>>> The cfs_rqs becomes throttled again, and migrate_tasks()
> > >>>> in migration_call skips their tasks (one more unthrottle
> > >>>> in migrate_tasks()->CPU_DYING does not happen, because rq->rd
> > >>>> is already NULL).
> > >>>>
> > >>>> Patch sets runtime_enabled to zero. This guarantees, the runtime
> > >>>> is not accounted, and the cfs_rqs won't exceed given
> > >>>> cfs_rq->runtime_remaining = 1, and tasks will be pickable
> > >>>> in migrate_tasks(). runtime_enabled is recalculated again
> > >>>> when rq becomes online again.
> > >>>>
> > >>>> Ben Segall also noticed, we always enable runtime in
> > >>>> tg_set_cfs_bandwidth(). Actually, we should do that for online
> > >>>> cpus only. To fix that, we check if a cpu is online when
> > >>>> its rq is locked. This guarantees we do not have races with
> > >>>> set_rq_offline(), which also requires rq->lock.
> > >>>>
> > >>>> v2: Fix race with tg_set_cfs_bandwidth().
> > >>>>     Move cfs_rq->runtime_enabled=0 above unthrottle_cfs_rq().
> > >>>>
> > >>>> Signed-off-by: Kirill Tkhai <ktkhai@parallels.com>
> > >>>> CC: Konstantin Khorenko <khorenko@parallels.com>
> > >>>> CC: Ben Segall <bsegall@google.com>
> > >>>> CC: Paul Turner <pjt@google.com>
> > >>>> CC: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> > >>>> CC: Mike Galbraith <umgwanakikbuti@gmail.com>
> > >>>> CC: Peter Zijlstra <peterz@infradead.org>
> > >>>> CC: Ingo Molnar <mingo@kernel.org>
> > >>>> ---
> > >>>>  kernel/sched/core.c |   15 +++++++++++----
> > >>>>  kernel/sched/fair.c |   22 ++++++++++++++++++++++
> > >>>>  2 files changed, 33 insertions(+), 4 deletions(-)
> > >>>>
> > >>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > >>>> index 7f3063c..707a3c5 100644
> > >>>> --- a/kernel/sched/core.c
> > >>>> +++ b/kernel/sched/core.c
> > >>>> @@ -7842,11 +7842,18 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
> > >>>>  		struct rq *rq = cfs_rq->rq;
> > >>>>  
> > >>>>  		raw_spin_lock_irq(&rq->lock);
> > >>>> -		cfs_rq->runtime_enabled = runtime_enabled;
> > >>>> -		cfs_rq->runtime_remaining = 0;
> > >>>> +		/*
> > >>>> +		 * Do not enable runtime on offline runqueues. We specially
> > >>>> +		 * make it disabled in unthrottle_offline_cfs_rqs().
> > >>>> +		 */
> > >>>> +		if (cpu_online(i)) {
> > >>>> +			cfs_rq->runtime_enabled = runtime_enabled;
> > >>>> +			cfs_rq->runtime_remaining = 0;
> > >>>> +
> > >>>> +			if (cfs_rq->throttled)
> > >>>> +				unthrottle_cfs_rq(cfs_rq);
> > >>>> +		}
> > >>>
> > >>> We can just do for_each_online_cpu, yes? Also we probably need
> > >>> get_online_cpus/put_online_cpus, and/or want cpu_active_mask instead
> > >>> right?
> > >>>
> > >>
> > >> Yes, we can use for_each_online_cpu/for_each_active_cpu with
> > >> get_online_cpus() taken. But it adds one more lock dependence.
> > >> This looks worse for me.
> > > 
> > > I mean, you need get_online_cpus anyway - cpu_online is just a test
> > > against the same mask that for_each_online_cpu uses, and without taking
> > > the lock you can still race with offlining and reset runtime_enabled.
> > > 
> > 
> > Oh, I see. Thanks.
> 
> But we can check for rq->online, don't we? How about this?

Oh, rq->online presents only in SMP case... Ok, lets do with
get_online_cpus()...

> 
>     sched/fair: Disable runtime_enabled on dying rq
>     
>     We kill rq->rd on the CPU_DOWN_PREPARE stage:
>     
>     	cpuset_cpu_inactive -> cpuset_update_active_cpus -> partition_sched_domains ->
>     	-> cpu_attach_domain -> rq_attach_root -> set_rq_offline
>     
>     This unthrottles all throttled cfs_rqs.
>     
>     But the cpu is still able to call schedule() till
>     
>     	take_cpu_down->__cpu_disable()
>     
>     is called from stop_machine.
>     
>     This case the tasks from just unthrottled cfs_rqs are pickable
>     in a standard scheduler way, and they are picked by dying cpu.
>     The cfs_rqs becomes throttled again, and migrate_tasks()
>     in migration_call skips their tasks (one more unthrottle
>     in migrate_tasks()->CPU_DYING does not happen, because rq->rd
>     is already NULL).
>     
>     Patch sets runtime_enabled to zero. This guarantees, the runtime
>     is not accounted, and the cfs_rqs won't exceed given
>     cfs_rq->runtime_remaining = 1, and tasks will be pickable
>     in migrate_tasks(). runtime_enabled is recalculated again
>     when rq becomes online again.
>     
>     Ben Segall also noticed, we always enable runtime in
>     tg_set_cfs_bandwidth(). Actually, we should do that for online
>     cpus only. To fix that, we check if a cpu is online when
>     its rq is locked. This guarantees we do not have races with
>     set_rq_offline(), which also requires rq->lock.
>     
>     v2: Fix race with tg_set_cfs_bandwidth().
>         Move cfs_rq->runtime_enabled=0 above unthrottle_cfs_rq().
>     v3: Check for rq->online instead of cpu_active.
>     
>     Signed-off-by: Kirill Tkhai <ktkhai@parallels.com>
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index ceea8d0..4c163c9 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7837,11 +7837,17 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
>  		struct rq *rq = cfs_rq->rq;
>  
>  		raw_spin_lock_irq(&rq->lock);
> -		cfs_rq->runtime_enabled = runtime_enabled;
> -		cfs_rq->runtime_remaining = 0;
> +		/*
> +		 * Do not enable runtime on offline runqueues. We specially
> +		 * disable it in unthrottle_offline_cfs_rqs().
> +		 */
> +		if (rq->online) {
> +			cfs_rq->runtime_enabled = runtime_enabled;
> +			cfs_rq->runtime_remaining = 0;
>  
> -		if (cfs_rq->throttled)
> -			unthrottle_cfs_rq(cfs_rq);
> +			if (cfs_rq->throttled)
> +				unthrottle_cfs_rq(cfs_rq);
> +		}
>  		raw_spin_unlock_irq(&rq->lock);
>  	}
>  	if (runtime_was_enabled && !runtime_enabled)
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1f9c457..5616d23 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3776,6 +3776,19 @@ static void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
>  	hrtimer_cancel(&cfs_b->slack_timer);
>  }
>  
> +static void __maybe_unused update_runtime_enabled(struct rq *rq)
> +{
> +	struct cfs_rq *cfs_rq;
> +
> +	for_each_leaf_cfs_rq(rq, cfs_rq) {
> +		struct cfs_bandwidth *cfs_b = &cfs_rq->tg->cfs_bandwidth;
> +
> +		raw_spin_lock(&cfs_b->lock);
> +		cfs_rq->runtime_enabled = cfs_b->quota != RUNTIME_INF;
> +		raw_spin_unlock(&cfs_b->lock);
> +	}
> +}
> +
>  static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
>  {
>  	struct cfs_rq *cfs_rq;
> @@ -3789,6 +3802,12 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
>  		 * there's some valid quota amount
>  		 */
>  		cfs_rq->runtime_remaining = 1;
> +		/*
> +		 * Offline rq is schedulable till cpu is completely disabled
> +		 * in take_cpu_down(), so we prevent new cfs throttling here.
> +		 */
> +		cfs_rq->runtime_enabled = 0;
> +
>  		if (cfs_rq_throttled(cfs_rq))
>  			unthrottle_cfs_rq(cfs_rq);
>  	}
> @@ -3832,6 +3851,7 @@ static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
>  	return NULL;
>  }
>  static inline void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b) {}
> +static inline void update_runtime_enabled(struct rq *rq) {}
>  static inline void unthrottle_offline_cfs_rqs(struct rq *rq) {}
>  
>  #endif /* CONFIG_CFS_BANDWIDTH */
> @@ -7325,6 +7345,8 @@ void trigger_load_balance(struct rq *rq)
>  static void rq_online_fair(struct rq *rq)
>  {
>  	update_sysctl();
> +
> +	update_runtime_enabled(rq);
>  }
>  
>  static void rq_offline_fair(struct rq *rq)
>