Re: [PATCH v5 4/4] futex: Avoid taking hb lock if nothing to wakeup

[Paul E. McKenney]

On Sat, Jan 11, 2014 at 01:49:12AM -0800, Paul E. McKenney wrote:
> On Thu, Jan 02, 2014 at 07:05:20AM -0800, Davidlohr Bueso wrote:
> > From: Davidlohr Bueso <davidlohr@xxxxxx>
> > 
> > In futex_wake() there is clearly no point in taking the hb->lock if we know
> > beforehand that there are no tasks to be woken. While the hash bucket's plist
> > head is a cheap way of knowing this, we cannot rely 100% on it as there is a
> > racy window between the futex_wait call and when the task is actually added to
> > the plist. To this end, we couple it with the spinlock check as tasks trying to
> > enter the critical region are most likely potential waiters that will be added
> > to the plist, thus preventing tasks sleeping forever if wakers don't acknowledge
> > all possible waiters.
> > 
> > Furthermore, the futex ordering guarantees are preserved, ensuring that waiters
> > either observe the changed user space value before blocking or is woken by a
> > concurrent waker. For wakers, this is done by relying on the barriers in
> > get_futex_key_refs() -- for archs that do have implicit mb in atomic_inc() we
> > explicitly add them through a new futex_get_mm function. For waiters we rely
> > on the fact that spin_lock calls already update the head counter, so spinners
> > are visible even if the lock hasn't been acquired yet.
> > 
> > For more details please refer to the updated comments in the code and related
> > discussion: https://lkml.org/lkml/2013/11/26/556
> > 
> > Special thanks to tglx for careful review and feedback.
> > 
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Darren Hart <dvhart@xxxxxxxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > Cc: Mike Galbraith <efault@xxxxxx>
> > Cc: Jeff Mahoney <jeffm@xxxxxxxx>
> > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Scott Norton <scott.norton@xxxxxx>
> > Cc: Tom Vaden <tom.vaden@xxxxxx>
> > Cc: Aswin Chandramouleeswaran <aswin@xxxxxx>
> > Cc: Waiman Long <Waiman.Long@xxxxxx>
> > Cc: Jason Low <jason.low2@xxxxxx>
> > Signed-off-by: Davidlohr Bueso <davidlohr@xxxxxx>
> 
> A couple of comments below.
> 
> 							Thanx, Paul
> 
> > ---
> >  kernel/futex.c | 113 +++++++++++++++++++++++++++++++++++++++++++++------------
> >  1 file changed, 90 insertions(+), 23 deletions(-)
> > 
> > diff --git a/kernel/futex.c b/kernel/futex.c
> > index fcc6850..5b4d09e 100644
> > --- a/kernel/futex.c
> > +++ b/kernel/futex.c
> > @@ -75,17 +75,20 @@
> >   * The waiter reads the futex value in user space and calls
> >   * futex_wait(). This function computes the hash bucket and acquires
> >   * the hash bucket lock. After that it reads the futex user space value
> > - * again and verifies that the data has not changed. If it has not
> > - * changed it enqueues itself into the hash bucket, releases the hash
> > - * bucket lock and schedules.
> > + * again and verifies that the data has not changed. If it has not changed
> > + * it enqueues itself into the hash bucket, releases the hash bucket lock
> > + * and schedules.
> >   *
> >   * The waker side modifies the user space value of the futex and calls
> > - * futex_wake(). This functions computes the hash bucket and acquires
> > - * the hash bucket lock. Then it looks for waiters on that futex in the
> > - * hash bucket and wakes them.
> > + * futex_wake(). This function computes the hash bucket and acquires the
> > + * hash bucket lock. Then it looks for waiters on that futex in the hash
> > + * bucket and wakes them.
> >   *
> > - * Note that the spin_lock serializes waiters and wakers, so that the
> > - * following scenario is avoided:
> > + * In scenarios where wakeups are called and no tasks are blocked on a futex,
> > + * taking the hb spinlock can be avoided and simply return. In order for this
> > + * optimization to work, ordering guarantees must exist so that the waiter
> > + * being added to the list is acknowledged when the list is concurrently being
> > + * checked by the waker, avoiding scenarios like the following:
> >   *
> >   * CPU 0                               CPU 1
> >   * val = *futex;
> > @@ -106,24 +109,50 @@
> >   * This would cause the waiter on CPU 0 to wait forever because it
> >   * missed the transition of the user space value from val to newval
> >   * and the waker did not find the waiter in the hash bucket queue.
> > - * The spinlock serializes that:
> > + *
> > + * The correct serialization ensures that a waiter either observes
> > + * the changed user space value before blocking or is woken by a
> > + * concurrent waker:
> >   *
> >   * CPU 0                               CPU 1
> >   * val = *futex;
> >   * sys_futex(WAIT, futex, val);
> >   *   futex_wait(futex, val);
> > - *   lock(hash_bucket(futex));
> > - *   uval = *futex;
> > - *                                     *futex = newval;
> > - *                                     sys_futex(WAKE, futex);
> > - *                                       futex_wake(futex);
> > - *                                       lock(hash_bucket(futex));
> > + *
> > + *   waiters++;
> > + *   mb(); (A) <-- paired with -.
> > + *                              |
> > + *   lock(hash_bucket(futex));  |
> > + *                              |
> > + *   uval = *futex;             |
> > + *                              |        *futex = newval;
> > + *                              |        sys_futex(WAKE, futex);
> > + *                              |          futex_wake(futex);
> > + *                              |
> > + *                              `------->   mb(); (B)
> >   *   if (uval == val)
> > - *      queue();
> > + *     queue();
> >   *     unlock(hash_bucket(futex));
> > - *     schedule();                       if (!queue_empty())
> > - *                                         wake_waiters(futex);
> > - *                                       unlock(hash_bucket(futex));
> > + *     schedule();                         if (waiters)
> > + *                                           lock(hash_bucket(futex));
> > + *                                           wake_waiters(futex);
> > + *                                           unlock(hash_bucket(futex));
> > + *
> > + * Where (A) orders the waiters increment and the futex value read -- this
> > + * is guaranteed by the head counter in the hb spinlock; and where (B)
> > + * orders the write to futex and the waiters read.
> > + *
> > + * This yields the following case (where X:=waiters, Y:=futex):
> > + *
> > + *	X = Y = 0
> > + *
> > + *	w[X]=1		w[Y]=1
> > + *	MB		MB
> > + *	r[Y]=y		r[X]=x
> > + *
> > + * Which guarantees that x==0 && y==0 is impossible; which translates back into
> > + * the guarantee that we cannot both miss the futex variable change and the
> > + * enqueue.
> >   */
> > 
> >  int __read_mostly futex_cmpxchg_enabled;
> > @@ -211,6 +240,38 @@ static unsigned long __read_mostly futex_hashsize;
> > 
> >  static struct futex_hash_bucket *futex_queues;
> > 
> > +static inline void futex_get_mm(union futex_key *key)
> > +{
> > +	atomic_inc(&key->private.mm->mm_count);
> > +#ifdef CONFIG_SMP
> 
> You don't need the #ifdef -- smp_mb__after_atomic_inc() does not emit
> code if !SMP.
> 
> > +	/*
> > +	 * Ensure futex_get_mm() implies a full barrier such that
> > +	 * get_futex_key() implies a full barrier. This is relied upon
> > +	 * as full barrier (B), see the ordering comment above.
> > +	 */
> > +	smp_mb__after_atomic_inc();
> > +#endif
> > +}
> > +
> > +static inline bool hb_waiters_pending(struct futex_hash_bucket *hb)
> > +{
> > +#ifdef CONFIG_SMP
> > +	/*
> > +	 * Tasks trying to enter the critical region are most likely
> > +	 * potential waiters that will be added to the plist. Ensure
> > +	 * that wakers won't miss to-be-slept tasks in the window between
> > +	 * the wait call and the actual plist_add.
> > +	 */
> > +	if (spin_is_locked(&hb->lock))
> > +		return true;
> > +	smp_rmb(); /* Make sure we check the lock state first */
> > +
> > +	return !plist_head_empty(&hb->chain);
> > +#else
> > +	return true;
> > +#endif
> > +}
> > +
> >  /*
> >   * We hash on the keys returned from get_futex_key (see below).
> >   */
> > @@ -245,10 +306,10 @@ static void get_futex_key_refs(union futex_key *key)
> > 
> >  	switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
> >  	case FUT_OFF_INODE:
> > -		ihold(key->shared.inode);
> > +		ihold(key->shared.inode); /* implies MB (B) */
> >  		break;
> >  	case FUT_OFF_MMSHARED:
> > -		atomic_inc(&key->private.mm->mm_count);
> > +		futex_get_mm(key); /* implies MB (B) */
> >  		break;
> >  	}
> >  }
> > @@ -322,7 +383,7 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw)
> >  	if (!fshared) {
> >  		key->private.mm = mm;
> >  		key->private.address = address;
> > -		get_futex_key_refs(key);
> > +		get_futex_key_refs(key);  /* implies MB (B) */
> >  		return 0;
> >  	}
> > 
> > @@ -1052,6 +1113,11 @@ futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
> >  		goto out;
> > 
> >  	hb = hash_futex(&key);
> > +
> > +	/* Make sure we really have tasks to wakeup */
> > +	if (!hb_waiters_pending(hb))
> > +		goto out_put_key;
> 
> Nice optimization, especially in the (hopefully) common case of low
> contention!
> 
> > +
> >  	spin_lock(&hb->lock);
> > 
> >  	plist_for_each_entry_safe(this, next, &hb->chain, list) {
> > @@ -1072,6 +1138,7 @@ futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
> >  	}
> > 
> >  	spin_unlock(&hb->lock);
> > +out_put_key:
> >  	put_futex_key(&key);
> >  out:
> >  	return ret;
> > @@ -1535,7 +1602,7 @@ static inline struct futex_hash_bucket *queue_lock(struct futex_q *q)
> >  	hb = hash_futex(&q->key);
> >  	q->lock_ptr = &hb->lock;
> > 
> > -	spin_lock(&hb->lock);
> > +	spin_lock(&hb->lock); /* implies MB (A) */
> 
> You need smp_mb__before_spinlock() before the spin_lock() to get a
> full memory barrier.

Actually, even that only gets you smp_mb().

Unless you are ordering a prior write against a later write here, you
will need an smp_mb().

							Thanx, Paul

> >  	return hb;
> >  }
> > 
> > -- 
> > 1.8.1.4
> > 

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/