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Commit 41ef8f82 authored by Yuanhan Liu's avatar Yuanhan Liu Committed by Ingo Molnar
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rwsem-spinlock: Implement writer lock-stealing for better scalability 

We (Linux Kernel Performance project) found a regression
introduced by commit:

  5a505085 mm/rmap: Convert the struct anon_vma::mutex to an rwsem

which converted all anon_vma::mutex locks rwsem write locks.

The semantics are the same, but the behavioral difference is
quite huge in some cases. After investigating it we found the
root cause: mutexes support lock stealing while rwsems don't.

Here is the link for the detailed regression report:

  https://lkml.org/lkml/2013/1/29/84



Ingo suggested adding write lock stealing to rwsems:

    "I think we should allow lock-steal between rwsem writers - that
     will not hurt fairness as most rwsem fairness concerns relate to
     reader vs. writer fairness"

And here is the rwsem-spinlock version.

With this patch, we got a double performance increase in one
test box with following aim7 workfile:

    FILESIZE: 1M
    POOLSIZE: 10M
    10 fork_test

 /usr/bin/time output w/o patch                       /usr/bin/time_output with patch
 -- Percent of CPU this job got: 369%                 Percent of CPU this job got: 537%
 Voluntary context switches: 640595016                Voluntary context switches: 157915561

We got a 45% increase in CPU usage and saved about 3/4 voluntary context switches.

Reported-by: default avatarLKP project <lkp@linux.intel.com>
Suggested-by: default avatarIngo Molnar <mingo@kernel.org>
Signed-off-by: default avatarYuanhan Liu <yuanhan.liu@linux.intel.com>
Cc: Alex Shi <alex.shi@intel.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Anton Blanchard <anton@samba.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: paul.gortmaker@windriver.com
Link: http://lkml.kernel.org/r/1359716356-23865-1-git-send-email-yuanhan.liu@linux.intel.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent fe2b05f7

lib/rwsem-spinlock.c

+24 −45
Original line number Diff line number Diff line
@@ -73,20 +73,13 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite) 
		goto dont_wake_writers;

	}



	/* if we are allowed to wake writers try to grant a single write lock

	 * if there's a writer at the front of the queue

	 * - we leave the 'waiting count' incremented to signify potential

	 *   contention

	/*

	 * as we support write lock stealing, we can't set sem->activity

	 * to -1 here to indicate we get the lock. Instead, we wake it up

	 * to let it go get it again.

	 */

	if (waiter->flags & RWSEM_WAITING_FOR_WRITE) {

		sem->activity = -1;

		list_del(&waiter->list);

		tsk = waiter->task;

		/* Don't touch waiter after ->task has been NULLed */

		smp_mb();

		waiter->task = NULL;

		wake_up_process(tsk);

		put_task_struct(tsk);

		wake_up_process(waiter->task);

		goto out;

	}
@@ -121,18 +114,10 @@ static inline struct rw_semaphore * 
__rwsem_wake_one_writer(struct rw_semaphore *sem)

{

	struct rwsem_waiter *waiter;

	struct task_struct *tsk;



	sem->activity = -1;



	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);

	list_del(&waiter->list);

	wake_up_process(waiter->task);



	tsk = waiter->task;

	smp_mb();

	waiter->task = NULL;

	wake_up_process(tsk);

	put_task_struct(tsk);

	return sem;

}
@@ -204,7 +189,6 @@ int __down_read_trylock(struct rw_semaphore *sem) 


/*

 * get a write lock on the semaphore

 * - we increment the waiting count anyway to indicate an exclusive lock

 */

void __sched __down_write_nested(struct rw_semaphore *sem, int subclass)

{
@@ -214,37 +198,32 @@ void __sched __down_write_nested(struct rw_semaphore *sem, int subclass) 


	raw_spin_lock_irqsave(&sem->wait_lock, flags);



	if (sem->activity == 0 && list_empty(&sem->wait_list)) {

		/* granted */

		sem->activity = -1;

		raw_spin_unlock_irqrestore(&sem->wait_lock, flags);

		goto out;

	}



	tsk = current;

	set_task_state(tsk, TASK_UNINTERRUPTIBLE);



	/* set up my own style of waitqueue */

	tsk = current;

	waiter.task = tsk;

	waiter.flags = RWSEM_WAITING_FOR_WRITE;

	get_task_struct(tsk);



	list_add_tail(&waiter.list, &sem->wait_list);



	/* we don't need to touch the semaphore struct anymore */

	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);



	/* wait to be given the lock */

	/* wait for someone to release the lock */

	for (;;) {

		if (!waiter.task)

		/*

		 * That is the key to support write lock stealing: allows the

		 * task already on CPU to get the lock soon rather than put

		 * itself into sleep and waiting for system woke it or someone

		 * else in the head of the wait list up.

		 */

		if (sem->activity == 0)

			break;

		schedule();

		set_task_state(tsk, TASK_UNINTERRUPTIBLE);

		raw_spin_unlock_irqrestore(&sem->wait_lock, flags);

		schedule();

		raw_spin_lock_irqsave(&sem->wait_lock, flags);

	}

	/* got the lock */

	sem->activity = -1;

	list_del(&waiter.list);



	tsk->state = TASK_RUNNING;

 out:

	;

	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);

}



void __sched __down_write(struct rw_semaphore *sem)
@@ -262,8 +241,8 @@ int __down_write_trylock(struct rw_semaphore *sem) 


	raw_spin_lock_irqsave(&sem->wait_lock, flags);



	if (sem->activity == 0 && list_empty(&sem->wait_list)) {

		/* granted */

	if (sem->activity == 0) {

		/* got the lock */

		sem->activity = -1;

		ret = 1;

	}