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Message-ID: <1403682814.3462.33.camel@tkhai>
Date: Wed, 25 Jun 2014 11:53:34 +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 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?
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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