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

[Kirill Tkhai <ktkhai@parallels.com>]

В Ср, 25/06/2014 в 09:52 -0700, bsegall@google.com пишет:
> Kirill Tkhai <ktkhai@parallels.com> writes:
> 
> > 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?
> 
> Yeah, that should work.

We can't base on it because rq->offline is not available in !SMP.
Could you review the message from [PATCH v3 1/3] topic?

> >
> >     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>
> 
> Reviewed-by: Ben Segall <bsegall@google.com>