[PATCH 4/6] sched: Optimize preemption operations on __schedule() callers

Frederic Weisbecker
2015-05-12T14:42:13Z

__schedule() disables preemption and some of its callers
(the preempt_schedule*() family) also set PREEMPT_ACTIVE.

So we have two preempt_count() modifications that could be performed
at once.

Lets remove the preemption disablement from __schedule() and pull
this responsibility to its callers in order to optimize preempt_count()
operations in a single place.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
---
include/linux/preempt.h | 12 ++++++++++++
kernel/sched/core.c | 29 +++++++++--------------------
2 files changed, 21 insertions(+), 20 deletions(-)

diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index 4689ef2..45da394 100644
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -137,6 +137,18 @@ extern void preempt_count_sub(int val);
#define preempt_count_inc() preempt_count_add(1)
#define preempt_count_dec() preempt_count_sub(1)

+#define preempt_active_enter() \
+do { \
+ preempt_count_add(PREEMPT_ACTIVE + PREEMPT_DISABLE_OFFSET); \
+ barrier(); \
+} while (0)
+
+#define preempt_active_exit() \
+do { \
+ barrier(); \
+ preempt_count_sub(PREEMPT_ACTIVE + PREEMPT_DISABLE_OFFSET); \
+} while (0)
+
#ifdef CONFIG_PREEMPT_COUNT

#define preempt_disable() \
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 8027cfd..b23def2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2723,9 +2723,7 @@ again:
* - return from syscall or exception to user-space
* - return from interrupt-handler to user-space
*
- * WARNING: all callers must re-check need_resched() afterward and reschedule
- * accordingly in case an event triggered the need for rescheduling (such as
- * an interrupt waking up a task) while preemption was disabled in __schedule().
+ * WARNING: must be called with preemption disabled!
*/
static void __sched __schedule(void)
{
@@ -2734,7 +2732,6 @@ static void __sched __schedule(void)
struct rq *rq;
int cpu;

- preempt_disable();
cpu = smp_processor_id();
rq = cpu_rq(cpu);
rcu_note_context_switch();
@@ -2798,8 +2795,6 @@ static void __sched __schedule(void)
raw_spin_unlock_irq(&rq->lock);

post_schedule(rq);
-
- sched_preempt_enable_no_resched();
}

static inline void sched_submit_work(struct task_struct *tsk)
@@ -2820,7 +2815,9 @@ asmlinkage __visible void __sched schedule(void)

sched_submit_work(tsk);
do {
+ preempt_disable();
__schedule();
+ sched_preempt_enable_no_resched();
} while (need_resched());
}
EXPORT_SYMBOL(schedule);
@@ -2859,15 +2856,14 @@ void __sched schedule_preempt_disabled(void)
static void __sched notrace preempt_schedule_common(void)
{
do {
- __preempt_count_add(PREEMPT_ACTIVE);
+ preempt_active_enter();
__schedule();
- __preempt_count_sub(PREEMPT_ACTIVE);
+ preempt_active_exit();

/*
* Check again in case we missed a preemption opportunity
* between schedule and now.
*/
- barrier();
} while (need_resched());
}

@@ -2914,7 +2910,7 @@ asmlinkage __visible void __sched notrace preempt_schedule_context(void)
return;

do {
- __preempt_count_add(PREEMPT_ACTIVE);
+ preempt_active_enter();
/*
* Needs preempt disabled in case user_exit() is traced
* and the tracer calls preempt_enable_notrace() causing
@@ -2924,8 +2920,7 @@ asmlinkage __visible void __sched notrace preempt_schedule_context(void)
__schedule();
exception_exit(prev_ctx);

- __preempt_count_sub(PREEMPT_ACTIVE);
- barrier();
+ preempt_active_exit();
} while (need_resched());
}
EXPORT_SYMBOL_GPL(preempt_schedule_context);
@@ -2949,17 +2944,11 @@ asmlinkage __visible void __sched preempt_schedule_irq(void)
prev_state = exception_enter();

do {
- __preempt_count_add(PREEMPT_ACTIVE);
+ preempt_active_enter();
local_irq_enable();
__schedule();
local_irq_disable();
- __preempt_count_sub(PREEMPT_ACTIVE);
-
- /*
- * Check again in case we missed a preemption opportunity
- * between schedule and now.
- */
- barrier();
+ preempt_active_exit();
} while (need_resched());

exception_exit(prev_state);
--
2.1.4