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Message-ID: <20171218193002.zzuocnd2hyt34ok5@hirez.programming.kicks-ass.net>
Date: Mon, 18 Dec 2017 20:30:02 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: rodrigosiqueira <rodrigosiqueiramelo@...il.com>,
Ingo Molnar <mingo@...nel.org>
Cc: kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] Adjustments: lock/unlock task in context_switch
Thanks; I've slightly changed it, find below. I'll queue it for the next
merge window.
---
Subject: sched: Rework / clarify prepare_lock_switch()
From: rodrigosiqueira <rodrigosiqueiramelo@...il.com>
Date: Fri, 15 Dec 2017 12:06:03 -0200
The function prepare_lock_switch has an unused parameter, and also the
function name was not descriptive. To improve the readability and remove
the extra parameter, the following changes were made:
* Moved prepare_lock_switch from kernel/sched/sched.h to
kernel/sched/core.c, renamed it to acquire_task, and removed the
unused parameter.
* Split the smp_store_release() out from finish_lock_switch() to a
function named release_task.
* Comments ajdustments.
Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: http://lkml.kernel.org/r/20171215140603.gxe5i2y6fg5ojfpp@smtp.gmail.com
---
kernel/sched/core.c | 53 +++++++++++++++++++++++++++++++++++++++++++++++----
kernel/sched/sched.h | 41 ---------------------------------------
2 files changed, 49 insertions(+), 45 deletions(-)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2045,7 +2045,7 @@ try_to_wake_up(struct task_struct *p, un
* If the owning (remote) CPU is still in the middle of schedule() with
* this task as prev, wait until its done referencing the task.
*
- * Pairs with the smp_store_release() in finish_lock_switch().
+ * Pairs with the smp_store_release() in release_task().
*
* This ensures that tasks getting woken will be fully ordered against
* their previous state and preserve Program Order.
@@ -2571,6 +2571,50 @@ fire_sched_out_preempt_notifiers(struct
#endif /* CONFIG_PREEMPT_NOTIFIERS */
+static inline void acquire_task(struct task_struct *next)
+{
+#ifdef CONFIG_SMP
+ /*
+ * Claim the task as running, we do this before switching to it
+ * such that any running task will have this set.
+ */
+ next->on_cpu = 1;
+#endif
+}
+
+static inline void release_task(struct task_struct *prev)
+{
+#ifdef CONFIG_SMP
+ /*
+ * After ->on_cpu is cleared, the task can be moved to a different CPU.
+ * We must ensure this doesn't happen until the switch is completely
+ * finished.
+ *
+ * In particular, the load of prev->state in finish_task_switch() must
+ * happen before this.
+ *
+ * Pairs with the smp_cond_load_acquire() in try_to_wake_up().
+ */
+ smp_store_release(&prev->on_cpu, 0);
+#endif
+}
+
+static inline void finish_lock_switch(struct rq *rq)
+{
+#ifdef CONFIG_DEBUG_SPINLOCK
+ /* this is a valid case when another task releases the spinlock */
+ rq->lock.owner = current;
+#endif
+ /*
+ * If we are tracking spinlock dependencies then we have to
+ * fix up the runqueue lock - which gets 'carried over' from
+ * prev into current:
+ */
+ spin_acquire(&rq->lock.dep_map, 0, 0, _THIS_IP_);
+
+ raw_spin_unlock_irq(&rq->lock);
+}
+
/**
* prepare_task_switch - prepare to switch tasks
* @rq: the runqueue preparing to switch
@@ -2591,7 +2635,7 @@ prepare_task_switch(struct rq *rq, struc
sched_info_switch(rq, prev, next);
perf_event_task_sched_out(prev, next);
fire_sched_out_preempt_notifiers(prev, next);
- prepare_lock_switch(rq, next);
+ acquire_task(next);
prepare_arch_switch(next);
}
@@ -2646,7 +2690,7 @@ static struct rq *finish_task_switch(str
* the scheduled task must drop that reference.
*
* We must observe prev->state before clearing prev->on_cpu (in
- * finish_lock_switch), otherwise a concurrent wakeup can get prev
+ * release_task), otherwise a concurrent wakeup can get prev
* running on another CPU and we could rave with its RUNNING -> DEAD
* transition, resulting in a double drop.
*/
@@ -2663,7 +2707,8 @@ static struct rq *finish_task_switch(str
* to use.
*/
smp_mb__after_unlock_lock();
- finish_lock_switch(rq, prev);
+ release_task(prev);
+ finish_lock_switch(rq);
finish_arch_post_lock_switch();
fire_sched_in_preempt_notifiers(current);
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1328,47 +1328,6 @@ static inline int task_on_rq_migrating(s
# define finish_arch_post_lock_switch() do { } while (0)
#endif
-static inline void prepare_lock_switch(struct rq *rq, struct task_struct *next)
-{
-#ifdef CONFIG_SMP
- /*
- * We can optimise this out completely for !SMP, because the
- * SMP rebalancing from interrupt is the only thing that cares
- * here.
- */
- next->on_cpu = 1;
-#endif
-}
-
-static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev)
-{
-#ifdef CONFIG_SMP
- /*
- * After ->on_cpu is cleared, the task can be moved to a different CPU.
- * We must ensure this doesn't happen until the switch is completely
- * finished.
- *
- * In particular, the load of prev->state in finish_task_switch() must
- * happen before this.
- *
- * Pairs with the smp_cond_load_acquire() in try_to_wake_up().
- */
- smp_store_release(&prev->on_cpu, 0);
-#endif
-#ifdef CONFIG_DEBUG_SPINLOCK
- /* this is a valid case when another task releases the spinlock */
- rq->lock.owner = current;
-#endif
- /*
- * If we are tracking spinlock dependencies then we have to
- * fix up the runqueue lock - which gets 'carried over' from
- * prev into current:
- */
- spin_acquire(&rq->lock.dep_map, 0, 0, _THIS_IP_);
-
- raw_spin_unlock_irq(&rq->lock);
-}
-
/*
* wake flags
*/
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