sched: Add NEED_RESCHED to the preempt_count
In order to combine the preemption and need_resched test we need to
fold the need_resched information into the preempt_count value.
Since the NEED_RESCHED flag is set across CPUs this needs to be an
atomic operation, however we very much want to avoid making
preempt_count atomic, therefore we keep the existing TIF_NEED_RESCHED
infrastructure in place but at 3 sites test it and fold its value into
preempt_count; namely:
- resched_task() when setting TIF_NEED_RESCHED on the current task
- scheduler_ipi() when resched_task() sets TIF_NEED_RESCHED on a
remote task it follows it up with a reschedule IPI
and we can modify the cpu local preempt_count from
there.
- cpu_idle_loop() for when resched_task() found tsk_is_polling().
We use an inverted bitmask to indicate need_resched so that a 0 means
both need_resched and !atomic.
Also remove the barrier() in preempt_enable() between
preempt_enable_no_resched() and preempt_check_resched() to avoid
having to reload the preemption value and allow the compiler to use
the flags of the previuos decrement. I couldn't come up with any sane
reason for this barrier() to be there as preempt_enable_no_resched()
already has a barrier() before doing the decrement.
Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-7a7m5qqbn5pmwnd4wko9u6da@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index fe89afa..ee61f5a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -525,8 +525,10 @@
set_tsk_need_resched(p);
cpu = task_cpu(p);
- if (cpu == smp_processor_id())
+ if (cpu == smp_processor_id()) {
+ set_preempt_need_resched();
return;
+ }
/* NEED_RESCHED must be visible before we test polling */
smp_mb();
@@ -1391,6 +1393,14 @@
void scheduler_ipi(void)
{
+ /*
+ * Fold TIF_NEED_RESCHED into the preempt_count; anybody setting
+ * TIF_NEED_RESCHED remotely (for the first time) will also send
+ * this IPI.
+ */
+ if (tif_need_resched())
+ set_preempt_need_resched();
+
if (llist_empty(&this_rq()->wake_list)
&& !tick_nohz_full_cpu(smp_processor_id())
&& !got_nohz_idle_kick())
@@ -1714,7 +1724,7 @@
#endif
#ifdef CONFIG_PREEMPT_COUNT
/* Want to start with kernel preemption disabled. */
- task_thread_info(p)->preempt_count = 1;
+ task_thread_info(p)->preempt_count = PREEMPT_DISABLED;
#endif
#ifdef CONFIG_SMP
plist_node_init(&p->pushable_tasks, MAX_PRIO);
@@ -2425,6 +2435,7 @@
put_prev_task(rq, prev);
next = pick_next_task(rq);
clear_tsk_need_resched(prev);
+ clear_preempt_need_resched();
rq->skip_clock_update = 0;
if (likely(prev != next)) {
@@ -2536,11 +2547,10 @@
*/
asmlinkage void __sched preempt_schedule_irq(void)
{
- struct thread_info *ti = current_thread_info();
enum ctx_state prev_state;
/* Catch callers which need to be fixed */
- BUG_ON(ti->preempt_count || !irqs_disabled());
+ BUG_ON(preempt_count() || !irqs_disabled());
prev_state = exception_enter();
@@ -4207,7 +4217,7 @@
raw_spin_unlock_irqrestore(&rq->lock, flags);
/* Set the preempt count _outside_ the spinlocks! */
- task_thread_info(idle)->preempt_count = 0;
+ task_thread_info(idle)->preempt_count = PREEMPT_ENABLED;
/*
* The idle tasks have their own, simple scheduling class:
