| From 527cb2f3b9b6a96b8dd6cd40abcaffc315bc95a2 Mon Sep 17 00:00:00 2001 |
| From: Gregory Haskins <ghaskins@novell.com> |
| Date: Fri, 3 Jul 2009 08:44:21 -0500 |
| Subject: [PATCH] rtmutex: Optimize rt lock wakeup |
| |
| commit 5aef092b9e9aeac49cd2541cf2913663569c86ce in tip. |
| |
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| |
| It is redundant to wake the grantee task if it is already running, and |
| the call to wake_up_process is relatively expensive. If we can safely |
| skip it we can measurably improve the performance of the adaptive-locks. |
| |
| Credit goes to Peter Morreale for the general idea. |
| |
| Signed-off-by: Gregory Haskins <ghaskins@novell.com> |
| Signed-off-by: Peter Morreale <pmorreale@novell.com> |
| Signed-off-by: Ingo Molnar <mingo@elte.hu> |
| Signed-off-by: Thomas Gleixner <tglx@linutronix.de> |
| |
| diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c |
| index be9864f..6435b54 100644 |
| --- a/kernel/rtmutex.c |
| +++ b/kernel/rtmutex.c |
| @@ -523,6 +523,41 @@ static void wakeup_next_waiter(struct rt_mutex *lock, int savestate) |
| pendowner = waiter->task; |
| waiter->task = NULL; |
| |
| + /* |
| + * Do the wakeup before the ownership change to give any spinning |
| + * waiter grantees a headstart over the other threads that will |
| + * trigger once owner changes. |
| + */ |
| + 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); |
| + } |
| + |
| rt_mutex_set_owner(lock, pendowner, RT_MUTEX_OWNER_PENDING); |
| |
| raw_spin_unlock(¤t->pi_lock); |
| @@ -549,11 +584,6 @@ static void wakeup_next_waiter(struct rt_mutex *lock, int savestate) |
| plist_add(&next->pi_list_entry, &pendowner->pi_waiters); |
| } |
| raw_spin_unlock(&pendowner->pi_lock); |
| - |
| - if (savestate) |
| - wake_up_process_mutex(pendowner); |
| - else |
| - wake_up_process(pendowner); |
| } |
| |
| /* |
| @@ -791,6 +821,11 @@ rt_spin_lock_slowlock(struct rt_mutex *lock) |
| |
| if (adaptive_wait(&waiter, orig_owner)) { |
| update_current(TASK_UNINTERRUPTIBLE, &saved_state); |
| + /* |
| + * The xchg() in update_current() is an implicit |
| + * barrier which we rely upon to ensure current->state |
| + * is visible before we test waiter.task. |
| + */ |
| if (waiter.task) |
| schedule_rt_mutex(lock); |
| } |
| -- |
| 1.7.1.1 |
| |