From: Steven Rostedt <srostedt@redhat.com>
The commit: rtmutex: Optimize rt lock wakeup
Does not do what it was suppose to do.
This is because the adaptive waiter sets its state to TASK_(UN)INTERRUPTIBLE
before going into the loop. Thus, the test in wakeup_next_waiter()
will always fail on an adaptive waiter, as it only tests to see if
the pending waiter never has its state set ot TASK_RUNNING unless
something else had woke it up.
The smp_mb() added to make this test work is just as expensive as
just calling wakeup. And since we we fail to wake up anyway, we are
doing both a smp_mb() and wakeup as well.
I tested this with dbench and we run faster without this patch.
I also tried a variant that instead fixed the loop, to change the state
only if the spinner was to go to sleep, and that still did not show
any improvement.
Cc: Gregory Haskins <ghaskins@novell.com>
Cc: Peter Morreale <pmorreale@novell.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
kernel/rtmutex.c | 29 ++---------------------------
1 files changed, 2 insertions(+), 27 deletions(-)
diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index 318d7ed..e218873 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -554,33 +554,8 @@ static void wakeup_next_waiter(struct rt_mutex *lock, int savestate)
*/
if (!savestate)
wake_up_process(pendowner);
- else {
- /*
- * We can skip the actual (expensive) wakeup if the
- * waiter is already running, but we have to be careful
- * of race conditions because they may be about to sleep.
- *
- * The waiter-side protocol has the following pattern:
- * 1: Set state != RUNNING
- * 2: Conditionally sleep if waiter->task != NULL;
- *
- * And the owner-side has the following:
- * A: Set waiter->task = NULL
- * B: Conditionally wake if the state != RUNNING
- *
- * As long as we ensure 1->2 order, and A->B order, we
- * will never miss a wakeup.
- *
- * Therefore, this barrier ensures that waiter->task = NULL
- * is visible before we test the pendowner->state. The
- * corresponding barrier is in the sleep logic.
- */
- smp_mb();
-
- /* If !RUNNING && !RUNNING_MUTEX */
- if (pendowner->state & ~TASK_RUNNING_MUTEX)
- wake_up_process_mutex(pendowner);
- }
+ else
+ wake_up_process_mutex(pendowner);
rt_mutex_set_owner(lock, pendowner, RT_MUTEX_OWNER_PENDING);
--
1.7.2.3
--
