From 5ef4421d52dc90ab4ad61080e2ea1c23d81c1cf9 Mon Sep 17 00:00:00 2001 From: Gregory Haskins Date: Fri, 3 Jul 2009 08:44:21 -0500 Subject: [PATCH] rtmutex: Optimize rt lock wakeup commit 5aef092b9e9aeac49cd2541cf2913663569c86ce in tip. [ The following text is in the "utf-8" character set. ] [ Your display is set for the "iso-8859-1" character set. ] [ Some characters may be displayed incorrectly. ] 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 Signed-off-by: Peter Morreale Signed-off-by: Ingo Molnar Signed-off-by: Thomas Gleixner Signed-off-by: Paul Gortmaker --- kernel/rtmutex.c | 45 ++++++++++++++++++++++++++++++++++++++++----- 1 files changed, 40 insertions(+), 5 deletions(-) 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); } -- 1.7.0.4