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Message-ID: <20140111095236.GA1181@linux.vnet.ibm.com>
Date: Sat, 11 Jan 2014 01:52:36 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Davidlohr Bueso <davidlohr@...com>
Cc: linux-kernel@...r.kernel.org, mingo@...nel.org,
dvhart@...ux.intel.com, peterz@...radead.org, tglx@...utronix.de,
efault@....de, jeffm@...e.com, torvalds@...ux-foundation.org,
jason.low2@...com, Waiman.Long@...com, tom.vaden@...com,
scott.norton@...com, aswin@...com
Subject: Re: [PATCH v5 4/4] futex: Avoid taking hb lock if nothing to wakeup
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@...com>
> >
> > 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@...nel.org>
> > Cc: Darren Hart <dvhart@...ux.intel.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Cc: Mike Galbraith <efault@....de>
> > Cc: Jeff Mahoney <jeffm@...e.com>
> > Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> > Cc: Scott Norton <scott.norton@...com>
> > Cc: Tom Vaden <tom.vaden@...com>
> > Cc: Aswin Chandramouleeswaran <aswin@...com>
> > Cc: Waiman Long <Waiman.Long@...com>
> > Cc: Jason Low <jason.low2@...com>
> > Signed-off-by: Davidlohr Bueso <davidlohr@...com>
>
> 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
> >
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