locking/mutex: Add lock handoff to avoid starvation
Implement lock handoff to avoid lock starvation. Lock starvation is possible because mutex_lock() allows lock stealing, where a running (or optimistic spinning) task beats the woken waiter to the acquire. Lock stealing is an important performance optimization because waiting for a waiter to wake up and get runtime can take a significant time, during which everyboy would stall on the lock. The down-side is of course that it allows for starvation. This patch has the waiter requesting a handoff if it fails to acquire the lock upon waking. This re-introduces some of the wait time, because once we do a handoff we have to wait for the waiter to wake up again. A future patch will add a round of optimistic spinning to attempt to alleviate this penalty, but if that turns out to not be enough, we can add a counter and only request handoff after multiple failed wakeups. There are a few tricky implementation details: - accepting a handoff must only be done in the wait-loop. Since the handoff condition is owner == current, it can easily cause recursive locking trouble. - accepting the handoff must be careful to provide the ACQUIRE semantics. - having the HANDOFF bit set on unlock requires care, we must not clear the owner. - we must be careful to not leave HANDOFF set after we've acquired the lock. The tricky scenario is setting the HANDOFF bit on an unlocked mutex. Tested-by: Jason Low <jason.low2@hpe.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Waiman Long <Waiman.Long@hpe.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: linux-kernel@vger.kernel.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra
Ingo Molnar
parent
a3ea3d9b86
commit
9d659ae14b
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@ -54,8 +54,10 @@ EXPORT_SYMBOL(__mutex_init);
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* bits to store extra state.
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*
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* Bit0 indicates a non-empty waiter list; unlock must issue a wakeup.
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* Bit1 indicates unlock needs to hand the lock to the top-waiter
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*/
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#define MUTEX_FLAG_WAITERS 0x01
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#define MUTEX_FLAG_HANDOFF 0x02
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#define MUTEX_FLAGS 0x03
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@ -71,20 +73,48 @@ static inline unsigned long __owner_flags(unsigned long owner)
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/*
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* Actual trylock that will work on any unlocked state.
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*
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* When setting the owner field, we must preserve the low flag bits.
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*
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* Be careful with @handoff, only set that in a wait-loop (where you set
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* HANDOFF) to avoid recursive lock attempts.
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*/
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static inline bool __mutex_trylock(struct mutex *lock)
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static inline bool __mutex_trylock(struct mutex *lock, const bool handoff)
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{
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unsigned long owner, curr = (unsigned long)current;
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owner = atomic_long_read(&lock->owner);
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for (;;) { /* must loop, can race against a flag */
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unsigned long old;
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unsigned long old, flags = __owner_flags(owner);
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if (__owner_task(owner)) {
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if (handoff && unlikely(__owner_task(owner) == current)) {
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/*
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* Provide ACQUIRE semantics for the lock-handoff.
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*
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* We cannot easily use load-acquire here, since
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* the actual load is a failed cmpxchg, which
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* doesn't imply any barriers.
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*
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* Also, this is a fairly unlikely scenario, and
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* this contains the cost.
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*/
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smp_mb(); /* ACQUIRE */
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return true;
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}
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if (__owner_task(owner))
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return false;
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}
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old = atomic_long_cmpxchg_acquire(&lock->owner, owner,
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curr | __owner_flags(owner));
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/*
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* We set the HANDOFF bit, we must make sure it doesn't live
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* past the point where we acquire it. This would be possible
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* if we (accidentally) set the bit on an unlocked mutex.
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*/
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if (handoff)
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flags &= ~MUTEX_FLAG_HANDOFF;
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old = atomic_long_cmpxchg_acquire(&lock->owner, owner, curr | flags);
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if (old == owner)
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return true;
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@ -134,6 +164,39 @@ static inline void __mutex_clear_flag(struct mutex *lock, unsigned long flag)
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atomic_long_andnot(flag, &lock->owner);
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}
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static inline bool __mutex_waiter_is_first(struct mutex *lock, struct mutex_waiter *waiter)
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{
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return list_first_entry(&lock->wait_list, struct mutex_waiter, list) == waiter;
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}
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/*
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* Give up ownership to a specific task, when @task = NULL, this is equivalent
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* to a regular unlock. Clears HANDOFF, preserves WAITERS. Provides RELEASE
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* semantics like a regular unlock, the __mutex_trylock() provides matching
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* ACQUIRE semantics for the handoff.
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*/
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static void __mutex_handoff(struct mutex *lock, struct task_struct *task)
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{
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unsigned long owner = atomic_long_read(&lock->owner);
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for (;;) {
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unsigned long old, new;
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#ifdef CONFIG_DEBUG_MUTEXES
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DEBUG_LOCKS_WARN_ON(__owner_task(owner) != current);
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#endif
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new = (owner & MUTEX_FLAG_WAITERS);
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new |= (unsigned long)task;
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old = atomic_long_cmpxchg_release(&lock->owner, owner, new);
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if (old == owner)
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break;
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owner = old;
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}
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}
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#ifndef CONFIG_DEBUG_LOCK_ALLOC
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/*
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* We split the mutex lock/unlock logic into separate fastpath and
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@ -398,7 +461,7 @@ static bool mutex_optimistic_spin(struct mutex *lock,
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break;
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/* Try to acquire the mutex if it is unlocked. */
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if (__mutex_trylock(lock)) {
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if (__mutex_trylock(lock, false)) {
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osq_unlock(&lock->osq);
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return true;
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}
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@ -523,6 +586,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
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struct task_struct *task = current;
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struct mutex_waiter waiter;
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unsigned long flags;
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bool first = false;
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int ret;
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if (use_ww_ctx) {
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@ -534,7 +598,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
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preempt_disable();
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mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);
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if (__mutex_trylock(lock) || mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx)) {
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if (__mutex_trylock(lock, false) ||
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mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx)) {
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/* got the lock, yay! */
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lock_acquired(&lock->dep_map, ip);
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if (use_ww_ctx) {
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@ -551,7 +616,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
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/*
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* After waiting to acquire the wait_lock, try again.
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*/
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if (__mutex_trylock(lock))
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if (__mutex_trylock(lock, false))
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goto skip_wait;
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debug_mutex_lock_common(lock, &waiter);
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@ -561,13 +626,13 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
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list_add_tail(&waiter.list, &lock->wait_list);
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waiter.task = task;
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if (list_first_entry(&lock->wait_list, struct mutex_waiter, list) == &waiter)
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if (__mutex_waiter_is_first(lock, &waiter))
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__mutex_set_flag(lock, MUTEX_FLAG_WAITERS);
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lock_contended(&lock->dep_map, ip);
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for (;;) {
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if (__mutex_trylock(lock))
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if (__mutex_trylock(lock, first))
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break;
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/*
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@ -586,17 +651,20 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
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}
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__set_task_state(task, state);
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/* didn't get the lock, go to sleep: */
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spin_unlock_mutex(&lock->wait_lock, flags);
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schedule_preempt_disabled();
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spin_lock_mutex(&lock->wait_lock, flags);
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if (!first && __mutex_waiter_is_first(lock, &waiter)) {
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first = true;
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__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
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}
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}
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__set_task_state(task, TASK_RUNNING);
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mutex_remove_waiter(lock, &waiter, task);
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if (likely(list_empty(&lock->wait_list)))
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__mutex_clear_flag(lock, MUTEX_FLAG_WAITERS);
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__mutex_clear_flag(lock, MUTEX_FLAGS);
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debug_mutex_free_waiter(&waiter);
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@ -724,33 +792,61 @@ EXPORT_SYMBOL_GPL(__ww_mutex_lock_interruptible);
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*/
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static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigned long ip)
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{
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struct task_struct *next = NULL;
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unsigned long owner, flags;
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WAKE_Q(wake_q);
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mutex_release(&lock->dep_map, 1, ip);
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/*
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* Release the lock before (potentially) taking the spinlock
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* such that other contenders can get on with things ASAP.
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* Release the lock before (potentially) taking the spinlock such that
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* other contenders can get on with things ASAP.
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*
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* Except when HANDOFF, in that case we must not clear the owner field,
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* but instead set it to the top waiter.
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*/
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owner = atomic_long_fetch_and_release(MUTEX_FLAGS, &lock->owner);
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if (!__owner_flags(owner))
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return;
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owner = atomic_long_read(&lock->owner);
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for (;;) {
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unsigned long old;
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#ifdef CONFIG_DEBUG_MUTEXES
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DEBUG_LOCKS_WARN_ON(__owner_task(owner) != current);
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#endif
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if (owner & MUTEX_FLAG_HANDOFF)
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break;
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old = atomic_long_cmpxchg_release(&lock->owner, owner,
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__owner_flags(owner));
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if (old == owner) {
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if (owner & MUTEX_FLAG_WAITERS)
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break;
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return;
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}
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owner = old;
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}
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spin_lock_mutex(&lock->wait_lock, flags);
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debug_mutex_unlock(lock);
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if (!list_empty(&lock->wait_list)) {
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/* get the first entry from the wait-list: */
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struct mutex_waiter *waiter =
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list_entry(lock->wait_list.next,
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struct mutex_waiter, list);
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list_first_entry(&lock->wait_list,
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struct mutex_waiter, list);
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next = waiter->task;
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debug_mutex_wake_waiter(lock, waiter);
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wake_q_add(&wake_q, waiter->task);
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wake_q_add(&wake_q, next);
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}
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if (owner & MUTEX_FLAG_HANDOFF)
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__mutex_handoff(lock, next);
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spin_unlock_mutex(&lock->wait_lock, flags);
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wake_up_q(&wake_q);
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}
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@ -853,7 +949,7 @@ __ww_mutex_lock_interruptible_slowpath(struct ww_mutex *lock,
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*/
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int __sched mutex_trylock(struct mutex *lock)
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{
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bool locked = __mutex_trylock(lock);
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bool locked = __mutex_trylock(lock, false);
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if (locked)
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mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_);
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