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Message-ID: <xm26ionpyls8.fsf@sword-of-the-dawn.mtv.corp.google.com>
Date: Wed, 25 Jun 2014 09:52:07 -0700
From: bsegall@...gle.com
To: Kirill Tkhai <ktkhai@...allels.com>
Cc: <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>,
<tkhai@...dex.ru>
Subject: Re: [PATCH v2 1/3] sched/fair: Disable runtime_enabled on dying rq
Kirill Tkhai <ktkhai@...allels.com> writes:
> В Вт, 24/06/2014 в 23:26 +0400, Kirill Tkhai пишет:
>> On 24.06.2014 23:13, bsegall@...gle.com wrote:
>> > 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.
>> >
>>
>> Oh, I see. Thanks.
>
> But we can check for rq->online, don't we? How about this?
Yeah, that should work.
>
> 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@...allels.com>
Reviewed-by: Ben Segall <bsegall@...gle.com>
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