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Date: Wed, 25 Jun 2014 12:03:38 +0400
From: Kirill Tkhai <ktkhai@...allels.com>
To: Ben Segall <bsegall@...gle.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
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@...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?
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@...allels.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)
>
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