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Date: Thu, 23 Jul 2015 18:55:06 +0200
From: Frederic Weisbecker <fweisbec@...il.com>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
Christoph Lameter <cl@...ux.com>,
Ingo Molnar <mingo@...nel.org>,
Viresh Kumar <viresh.kumar@...aro.org>,
Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 07/10] sched: Migrate sched to use new tick dependency
mask model
On Thu, Jul 23, 2015 at 06:42:12PM +0200, Frederic Weisbecker wrote:
> Instead of providing asynchronous checks for the nohz subsystem to verify
> sched tick dependency, migrate sched to the new mask.
>
> The easiest is to recycle the current asynchronous tick dependency check
> which verifies the class of the current task and its requirements for
> periodic preemption checks.
>
> We need to evaluate this tick dependency on three places:
>
> 1) Task enqueue: One or more tasks have been enqueued, we must check
> if those are competing with the current task.
>
> 2) Task dequeue: A possibly competing task has been dequeued, clear the
> tick dependency if needed.
>
> 3) schedule(): we might be switching to a task of another scheduler
> class. Each class has its preemption rules, we must re-evaluate it.
>
> This doesn't change much compared to the previous layout, except that
> 3) has to be done with rq locked to avoid mask change racing with remote
> enqueue.
>
> We could get away with 3) by checking the highest prio tasks of the
> runqueue instead of its current task.
>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Preeti U Murthy <preeti@...ux.vnet.ibm.com>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Viresh Kumar <viresh.kumar@...aro.org>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> ---
> include/linux/sched.h | 3 ---
> kernel/sched/core.c | 12 ++++++-----
> kernel/sched/sched.h | 56 +++++++++++++++++++++++++++++++++++-------------
> kernel/time/tick-sched.c | 5 -----
> 4 files changed, 48 insertions(+), 28 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index ae21f15..88c99a2 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2296,10 +2296,7 @@ static inline void wake_up_nohz_cpu(int cpu) { }
> #endif
>
> #ifdef CONFIG_NO_HZ_FULL
> -extern bool sched_can_stop_tick(void);
> extern u64 scheduler_tick_max_deferment(void);
> -#else
> -static inline bool sched_can_stop_tick(void) { return false; }
> #endif
>
> #ifdef CONFIG_SCHED_AUTOGROUP
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 4d34035..6c3db36 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -714,21 +714,22 @@ static inline bool got_nohz_idle_kick(void)
> #endif /* CONFIG_NO_HZ_COMMON */
>
> #ifdef CONFIG_NO_HZ_FULL
> -bool sched_can_stop_tick(void)
> +bool sched_can_stop_tick(struct rq *rq)
> {
> + struct task_struct *curr = rq->curr;
> /*
> * FIFO realtime policy runs the highest priority task. Other runnable
> * tasks are of a lower priority. The scheduler tick does nothing.
> */
> - if (current->policy == SCHED_FIFO)
> + if (curr->policy == SCHED_FIFO)
> return true;
>
> /*
> * Round-robin realtime tasks time slice with other tasks at the same
> * realtime priority. Is this task the only one at this priority?
> */
> - if (current->policy == SCHED_RR) {
> - struct sched_rt_entity *rt_se = ¤t->rt;
> + if (curr->policy == SCHED_RR) {
> + struct sched_rt_entity *rt_se = &curr->rt;
>
> return rt_se->run_list.prev == rt_se->run_list.next;
> }
> @@ -738,7 +739,7 @@ bool sched_can_stop_tick(void)
> * nr_running update is assumed to be visible
> * after IPI is sent from wakers.
> */
> - if (this_rq()->nr_running > 1)
> + if (rq->nr_running > 1)
> return false;
>
> return true;
> @@ -2489,6 +2490,7 @@ static struct rq *finish_task_switch(struct task_struct *prev)
> put_task_struct(prev);
> }
>
> + sched_update_tick_dependency(rq);
A small mistake here that I forgot to update. This is supposed to be between
perf_event_task_sched_in() and finish_lock_switch(). This must be called
when rq is locked otherwise there is a risk that a remote CPU enqueues or
dequeues concurrently and mess up the dependency mask.
Here is the correct diff:
diff --git a/include/linux/sched.h b/include/linux/sched.h
index ae21f15..88c99a2 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2296,10 +2296,7 @@ static inline void wake_up_nohz_cpu(int cpu) { }
#endif
#ifdef CONFIG_NO_HZ_FULL
-extern bool sched_can_stop_tick(void);
extern u64 scheduler_tick_max_deferment(void);
-#else
-static inline bool sched_can_stop_tick(void) { return false; }
#endif
#ifdef CONFIG_SCHED_AUTOGROUP
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 4d34035..58b16d3 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -714,21 +714,22 @@ static inline bool got_nohz_idle_kick(void)
#endif /* CONFIG_NO_HZ_COMMON */
#ifdef CONFIG_NO_HZ_FULL
-bool sched_can_stop_tick(void)
+bool sched_can_stop_tick(struct rq *rq)
{
+ struct task_struct *curr = rq->curr;
/*
* FIFO realtime policy runs the highest priority task. Other runnable
* tasks are of a lower priority. The scheduler tick does nothing.
*/
- if (current->policy == SCHED_FIFO)
+ if (curr->policy == SCHED_FIFO)
return true;
/*
* Round-robin realtime tasks time slice with other tasks at the same
* realtime priority. Is this task the only one at this priority?
*/
- if (current->policy == SCHED_RR) {
- struct sched_rt_entity *rt_se = ¤t->rt;
+ if (curr->policy == SCHED_RR) {
+ struct sched_rt_entity *rt_se = &curr->rt;
return rt_se->run_list.prev == rt_se->run_list.next;
}
@@ -738,7 +739,7 @@ bool sched_can_stop_tick(void)
* nr_running update is assumed to be visible
* after IPI is sent from wakers.
*/
- if (this_rq()->nr_running > 1)
+ if (rq->nr_running > 1)
return false;
return true;
@@ -2471,6 +2472,7 @@ static struct rq *finish_task_switch(struct task_struct *prev)
vtime_task_switch(prev);
finish_arch_switch(prev);
perf_event_task_sched_in(prev, current);
+ sched_update_tick_dependency(rq);
finish_lock_switch(rq, prev);
finish_arch_post_lock_switch();
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 84d4879..5037acf 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1321,6 +1321,38 @@ unsigned long to_ratio(u64 period, u64 runtime);
extern void init_task_runnable_average(struct task_struct *p);
+#ifdef CONFIG_NO_HZ_FULL
+extern bool sched_can_stop_tick(struct rq *rq);
+
+/*
+ * Tick is needed if more than one task runs on a CPU.
+ * Send the target an IPI to kick it out of nohz mode.
+ *
+ * We assume that IPI implies full memory barrier and the
+ * new value of rq->nr_running is visible on reception
+ * from the target.
+ */
+static inline void sched_update_tick_dependency(struct rq *rq)
+{
+ int cpu;
+
+ if (!tick_nohz_full_enabled())
+ return;
+
+ cpu = cpu_of(rq);
+
+ if (!tick_nohz_full_cpu(rq->cpu))
+ return;
+
+ if (sched_can_stop_tick(rq))
+ tick_nohz_clear_tick_dependency_cpu(TICK_SCHED_BIT, cpu);
+ else
+ tick_nohz_set_tick_dependency_cpu(TICK_SCHED_BIT, cpu);
+}
+#else
+static inline void sched_update_tick_dependency(struct rq *rq) { }
+#endif
+
static inline void add_nr_running(struct rq *rq, unsigned count)
{
unsigned prev_nr = rq->nr_running;
@@ -1332,26 +1364,20 @@ static inline void add_nr_running(struct rq *rq, unsigned count)
if (!rq->rd->overload)
rq->rd->overload = true;
#endif
-
-#ifdef CONFIG_NO_HZ_FULL
- if (tick_nohz_full_cpu(rq->cpu)) {
- /*
- * Tick is needed if more than one task runs on a CPU.
- * Send the target an IPI to kick it out of nohz mode.
- *
- * We assume that IPI implies full memory barrier and the
- * new value of rq->nr_running is visible on reception
- * from the target.
- */
- tick_nohz_full_kick_cpu(rq->cpu);
- }
-#endif
+ /* Check if new task(s) need periodic preemption check */
+ sched_update_tick_dependency(rq);
}
}
static inline void sub_nr_running(struct rq *rq, unsigned count)
{
- rq->nr_running -= count;
+ unsigned prev_nr = rq->nr_running;
+
+ rq->nr_running = prev_nr - count;
+ if (prev_nr > 1) {
+ /* Check if we still need preemption */
+ sched_update_tick_dependency(rq);
+ }
}
static inline void rq_last_tick_reset(struct rq *rq)
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index fbe4736..e6447bd 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -192,11 +192,6 @@ static bool can_stop_full_tick(struct tick_sched *ts)
return false;
}
- if (!sched_can_stop_tick()) {
- trace_tick_stop(0, "more than 1 task in runqueue\n");
- return false;
- }
-
if (!posix_cpu_timers_can_stop_tick(current)) {
trace_tick_stop(0, "posix timers running\n");
return false;
--
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