locking/mutexes: Modify the way optimistic spinners are queued (47667fa1) · Commits · 戴 / test

The mutex->spin_mlock was introduced in order to ensure that only 1 thread spins for lock acquisition at a time to reduce cache line contention. When lock->owner is NULL and the lock->count is still not 1, the spinner(s) will continually release and obtain the lock->spin_mlock. This can generate quite a bit of overhead/contention, and also might just delay the spinner from getting the lock. This patch modifies the way optimistic spinners are queued by queuing before entering the optimistic spinning loop as oppose to acquiring before every call to mutex_spin_on_owner(). So in situations where the spinner requires a few extra spins before obtaining the lock, then there will only be 1 spinner trying to get the lock and it will avoid the overhead from unnecessarily unlocking and locking the spin_mlock. Signed-off-by:

Jason Low <jason.low2@hp.com> Cc: tglx@linutronix.de Cc: riel@redhat.com Cc: akpm@linux-foundation.org Cc: davidlohr@hp.com Cc: hpa@zytor.com Cc: andi@firstfloor.org Cc: aswin@hp.com Cc: scott.norton@hp.com Cc: chegu_vinod@hp.com Cc: Waiman.Long@hp.com Cc: paulmck@linux.vnet.ibm.com Cc: torvalds@linux-foundation.org Signed-off-by:

Peter Zijlstra <peterz@infradead.org> Link: http://lkml.kernel.org/r/1390936396-3962-3-git-send-email-jason.low2@hp.com Signed-off-by:

Ingo Molnar <mingo@kernel.org>

kernel/locking/mutex.c

+6 −9

Original line number	Diff line number	Diff line
		@@ -403,9 +403,9 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
		if (!mutex_can_spin_on_owner(lock))
		goto slowpath;

		mcs_spin_lock(&lock->mcs_lock, &node);
		for (;;) {
		struct task_struct *owner;
		struct mcs_spinlock node;

		if (use_ww_ctx && ww_ctx->acquired > 0) {
		struct ww_mutex *ww;
		@@ -420,19 +420,16 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
		* performed the optimistic spinning cannot be done.
		*/
		if (ACCESS_ONCE(ww->ctx))
		goto slowpath;
		break;
		}

		/*
		* If there's an owner, wait for it to either
		* release the lock or go to sleep.
		*/
		mcs_spin_lock(&lock->mcs_lock, &node);
		owner = ACCESS_ONCE(lock->owner);
		if (owner && !mutex_spin_on_owner(lock, owner)) {
		mcs_spin_unlock(&lock->mcs_lock, &node);
		goto slowpath;
		}
		if (owner && !mutex_spin_on_owner(lock, owner))
		break;

		if ((atomic_read(&lock->count) == 1) &&
		(atomic_cmpxchg(&lock->count, 1, 0) == 1)) {
		@@ -449,7 +446,6 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
		preempt_enable();
		return 0;
		}
		mcs_spin_unlock(&lock->mcs_lock, &node);

		/*
		* When there's no owner, we might have preempted between the
		@@ -458,7 +454,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
		* the owner complete.
		*/
		if (!owner && (need_resched() \|\| rt_task(task)))
		goto slowpath;
		break;

		/*
		* The cpu_relax() call is a compiler barrier which forces
		@@ -468,6 +464,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
		*/
		arch_mutex_cpu_relax();
		}
		mcs_spin_unlock(&lock->mcs_lock, &node);
		slowpath:
		#endif
		spin_lock_mutex(&lock->wait_lock, flags);

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