If an interrupt fires in cond_resched() between the call to __schedule()
and the PREEMPT_ACTIVE count decrementation with the interrupt having
set TIF_NEED_RESCHED, the call to preempt_schedule_irq() will be ignored
due to the PREEMPT_ACTIVE count. This kind of scenario, with irq preemption
being delayed because we are interrupting a preempt-disabled area, is
usually fixed up after preemption is re-enabled back with an explicit
call to preempt_schedule().
This is what preempt_enable() does but a raw preempt count decrement as
performed by __preempt_count_sub(PREEMPT_ACTIVE) doesn't handle delayed
preemption check. Therefore when such a race happens, the rescheduling
is going to be delayed until the next scheduler or preemption entrypoint.
This can be a problem for scheduler latency sensitive workloads.
Lets fix that by consolidating cond_resched() with preempt_schedule()
internals.
Reported-by: Linus Torvalds <torva...@linux-foundation.org>
Reported-and-inspired-by: Ingo Molnar <mi...@kernel.org>
Cc: Peter Zijlstra <pet...@infradead.org>
Signed-off-by: Frederic Weisbecker <fweis...@gmail.com>
---
kernel/sched/core.c | 40 +++++++++++++++++++---------------------
1 file changed, 19 insertions(+), 21 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b5797b7..069a2d8 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2877,6 +2877,21 @@ void __sched schedule_preempt_disabled(void)
preempt_disable();
}
+static void preempt_schedule_common(void)
+{
+ do {
+ __preempt_count_add(PREEMPT_ACTIVE);
+ __schedule();
+ __preempt_count_sub(PREEMPT_ACTIVE);
+
+ /*
+ * Check again in case we missed a preemption opportunity
+ * between schedule and now.
+ */
+ barrier();
+ } while (need_resched());
+}
+
#ifdef CONFIG_PREEMPT
/*
* this is the entry point to schedule() from in-kernel preemption
@@ -2892,17 +2907,7 @@ asmlinkage __visible void __sched notrace
preempt_schedule(void)
if (likely(!preemptible()))
return;
- do {
- __preempt_count_add(PREEMPT_ACTIVE);
- __schedule();
- __preempt_count_sub(PREEMPT_ACTIVE);
-
- /*
- * Check again in case we missed a preemption opportunity
- * between schedule and now.
- */
- barrier();
- } while (need_resched());
+ preempt_schedule_common();
}
NOKPROBE_SYMBOL(preempt_schedule);
EXPORT_SYMBOL(preempt_schedule);
@@ -4202,17 +4207,10 @@ SYSCALL_DEFINE0(sched_yield)
return 0;
}
-static void __cond_resched(void)
-{
- __preempt_count_add(PREEMPT_ACTIVE);
- __schedule();
- __preempt_count_sub(PREEMPT_ACTIVE);
-}
-
int __sched _cond_resched(void)
{
if (should_resched()) {
- __cond_resched();
+ preempt_schedule_common();
return 1;
}
return 0;
@@ -4237,7 +4235,7 @@ int __cond_resched_lock(spinlock_t *lock)
if (spin_needbreak(lock) || resched) {
spin_unlock(lock);
if (resched)
- __cond_resched();
+ preempt_schedule_common();
else
cpu_relax();
ret = 1;
@@ -4253,7 +4251,7 @@ int __sched __cond_resched_softirq(void)
if (should_resched()) {
local_bh_enable();
- __cond_resched();
+ preempt_schedule_common();
local_bh_disable();
return 1;
}
--
2.1.3
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