linux-kernel - Re: [PATCH v2 1/3] sched/fair: Disable runtime

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Date:	Tue, 24 Jun 2014 12:13:50 -0700
From:	bsegall@...gle.com
To:	tkhai@...dex.ru
Cc:	Kirill Tkhai <ktkhai@...allels.com>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	khorenko@...allels.com, Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH v2 1/3] sched/fair: Disable runtime_enabled on dying rq

Kirill Tkhai <tkhai@...dex.ru> writes:

> On 24.06.2014 21:03, bsegall@...gle.com wrote:
>> Kirill Tkhai <ktkhai@...allels.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@...allels.com>
>>> CC: Konstantin Khorenko <khorenko@...allels.com>
>>> CC: Ben Segall <bsegall@...gle.com>
>>> CC: Paul Turner <pjt@...gle.com>
>>> CC: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
>>> CC: Mike Galbraith <umgwanakikbuti@...il.com>
>>> CC: Peter Zijlstra <peterz@...radead.org>
>>> CC: Ingo Molnar <mingo@...nel.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.
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