From: Thomas Gleixner <tglx@linutronix.de>
To: LKML <linux-kernel@vger.kernel.org>
Cc: Sebastian Siewior <bigeasy@linutronix.de>,
Qais Yousef <qais.yousef@arm.com>, Ingo Molnar <mingo@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Juri Lelli <juri.lelli@redhat.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Steven Rostedt <rostedt@goodmis.org>,
Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
Valentin Schneider <valentin.schneider@arm.com>,
Scott Wood <swood@redhat.com>,
"Peter Zijlstra (Intel)" <peterz@infradead.org>
Subject: [patch 03/10] sched/core: Wait for tasks being pushed away on hotplug
Date: Thu, 17 Sep 2020 11:42:05 +0200 [thread overview]
Message-ID: <20200917101624.205718256@linutronix.de> (raw)
In-Reply-To: 20200917094202.301694311@linutronix.de
RT kernels need to ensure that all tasks which are not per CPU kthreads
have left the outgoing CPU to guarantee that no tasks are force migrated
within a migrate disabled section.
There is also some desire to (ab)use fine grained CPU hotplug control to
clear a CPU from active state to force migrate tasks which are not per CPU
kthreads away for power control purposes.
Add a mechanism which waits until all tasks which should leave the CPU
after the CPU active flag is cleared have moved to a different online CPU.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
Cc: Valentin Schneider <valentin.schneider@arm.com>
---
kernel/sched/core.c | 44 +++++++++++++++++++++++++++++++++++++++++---
kernel/sched/sched.h | 4 ++++
2 files changed, 45 insertions(+), 3 deletions(-)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6892,11 +6892,24 @@ static bool balance_push(struct rq *rq)
SCHED_WARN_ON(rq->cpu != smp_processor_id());
/*
- * Both the cpu-hotplug and stop task are in this case and are
+ * Both the cpu-hotplug and stop task are in this class and are
* required to complete the hotplug process.
*/
- if (is_per_cpu_kthread(push_task))
+ if (is_per_cpu_kthread(push_task)) {
+ /*
+ * If this is the idle task on the outgoing CPU try to wake
+ * up the hotplug control thread which might wait for the
+ * last task to vanish. The rcuwait_active() check is
+ * accurate here because the waiter is pinned on this CPU
+ * and can't obviously be running in parallel.
+ */
+ if (!rq->nr_running && rcuwait_active(&rq->hotplug_wait)) {
+ raw_spin_unlock(&rq->lock);
+ rcuwait_wake_up(&rq->hotplug_wait);
+ raw_spin_lock(&rq->lock);
+ }
return false;
+ }
get_task_struct(push_task);
/*
@@ -6929,13 +6942,31 @@ static void balance_push_set(int cpu, bo
rq_unlock_irqrestore(rq, &rf);
}
-#else
+/*
+ * Invoked from a CPUs hotplug control thread after the CPU has been marked
+ * inactive. All tasks which are not per CPU kernel threads are either
+ * pushed off this CPU now via balance_push() or placed on a different CPU
+ * during wakeup. Wait until the CPU is quiescent.
+ */
+static void balance_hotplug_wait(void)
+{
+ struct rq *rq = this_rq();
+
+ rcuwait_wait_event(&rq->hotplug_wait, rq->nr_running == 1,
+ TASK_UNINTERRUPTIBLE);
+}
+
+#else /* !CONFIG_HOTPLUG_CPU */
+
+static inline void balance_push_set(int cpu, bool on) { }
static inline bool balance_push(struct rq *rq)
{
return false;
}
+static inline void balance_hotplug_wait(void) { }
+
#endif /* CONFIG_HOTPLUG_CPU */
void set_rq_online(struct rq *rq)
@@ -7090,6 +7121,10 @@ int sched_cpu_deactivate(unsigned int cp
return ret;
}
sched_domains_numa_masks_clear(cpu);
+
+ /* Wait for all non per CPU kernel threads to vanish. */
+ balance_hotplug_wait();
+
return 0;
}
@@ -7330,6 +7365,9 @@ void __init sched_init(void)
rq_csd_init(rq, &rq->nohz_csd, nohz_csd_func);
#endif
+#ifdef CONFIG_HOTPLUG_CPU
+ rcuwait_init(&rq->hotplug_wait);
+#endif
#endif /* CONFIG_SMP */
hrtick_rq_init(rq);
atomic_set(&rq->nr_iowait, 0);
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1004,6 +1004,10 @@ struct rq {
/* This is used to determine avg_idle's max value */
u64 max_idle_balance_cost;
+
+#ifdef CONFIG_HOTPLUG_CPU
+ struct rcuwait hotplug_wait;
+#endif
#endif /* CONFIG_SMP */
#ifdef CONFIG_IRQ_TIME_ACCOUNTING
next prev parent reply other threads:[~2020-09-17 10:51 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-17 9:42 [patch 00/10] sched: Migrate disable support for RT Thomas Gleixner
2020-09-17 9:42 ` [patch 01/10] sched: Fix balance_callback() Thomas Gleixner
2020-09-17 9:42 ` [patch 02/10] sched/hotplug: Ensure only per-cpu kthreads run during hotplug Thomas Gleixner
2020-09-17 9:42 ` Thomas Gleixner [this message]
2020-09-17 9:42 ` [patch 04/10] sched/hotplug: Consolidate task migration on CPU unplug Thomas Gleixner
2020-09-17 9:42 ` [patch 05/10] sched/core: Split __set_cpus_allowed_ptr() Thomas Gleixner
2020-09-17 9:42 ` [patch 06/10] sched: Add task components for migration control Thomas Gleixner
2020-09-17 9:42 ` [patch 07/10] sched/core: Add mechanism to wait for affinity setting to complete Thomas Gleixner
2020-09-17 9:42 ` [patch 08/10] sched: Add update_migratory() callback to scheduler classes Thomas Gleixner
2020-09-17 9:42 ` [patch 09/10] sched/core: Add migrate_disable/enable() Thomas Gleixner
2020-09-17 14:24 ` peterz
2020-09-17 14:38 ` Sebastian Siewior
2020-09-17 14:49 ` peterz
2020-09-17 15:13 ` Sebastian Siewior
2020-09-17 15:54 ` peterz
2020-09-17 16:30 ` Sebastian Siewior
2020-09-18 8:22 ` peterz
2020-09-18 8:48 ` Sebastian Siewior
2020-09-18 7:00 ` Thomas Gleixner
2020-09-18 8:28 ` peterz
2020-09-17 9:42 ` [patch 10/10] sched/core: Make migrate disable and CPU hotplug cooperative Thomas Gleixner
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