sched: Fix missing preemption opportunity

If an interrupt fires in cond_resched(), between the call to __schedule()
and the PREEMPT_ACTIVE count decrementation, and that interrupt sets
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 it's 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 <torvalds@linux-foundation.org>
Reported-by: Ingo Molnar <mingo@kernel.org>
Original-patch-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1421946484-9298-1-git-send-email-fweisbec@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 19 insertions, 21 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0b591fe67b70..54dce019c0ce 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2884,6 +2884,21 @@ 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
@@ -2899,17 +2914,7 @@ 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);
@@ -4209,17 +4214,10 @@ 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;
@@ -4244,7 +4242,7 @@ 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;
@@ -4260,7 +4258,7 @@ int __sched __cond_resched_softirq(void)
 
         if (should_resched()) {
                 local_bh_enable();
-                __cond_resched();
+                preempt_schedule_common();
                 local_bh_disable();
                 return 1;
         }