Re: [PATCH 3/7] locking/rwsem: check for active writer/spinner before wakeup

[Waiman Long]

On 08/04/2014 05:20 PM, Jason Low wrote:
> On Sun, 2014-08-03 at 22:36 -0400, Waiman Long wrote:
> > On a highly contended rwsem, spinlock contention due to the slow
> > rwsem_wake() call can be a significant portion of the total CPU cycles
> > used. With writer lock stealing and writer optimistic spinning, there
> > is also a pretty good chance that the lock may have been stolen
> > before the waker wakes up the waiters. The woken tasks, if any,
> > will have to go back to sleep again.
> >
> > This patch adds checking code at the beginning of the rwsem_wake()
> > and __rwsem_do_wake() function to look for spinner and active
> > writer respectively.  The presence of an active writer will abort the
> > wakeup operation.  The presence of a spinner will still allow wakeup
> > operation to proceed as long as the trylock operation succeeds. This
> > strikes a good balance between excessive spinlock contention especially
> > when there are a lot of active readers and a lot of failed fastpath
> > operations because there are tasks waiting in the queue.
> >
> > Signed-off-by: Waiman Long<Waiman.Long@xxxxxx>
> > ---
> >   include/linux/osq_lock.h    |    5 ++++
> >   kernel/locking/rwsem-xadd.c |   57 ++++++++++++++++++++++++++++++++++++++++++-
> >   2 files changed, 61 insertions(+), 1 deletions(-)
> >
> > diff --git a/include/linux/osq_lock.h b/include/linux/osq_lock.h
> > index 90230d5..79db546 100644
> > --- a/include/linux/osq_lock.h
> > +++ b/include/linux/osq_lock.h
> > @@ -24,4 +24,9 @@ static inline void osq_lock_init(struct optimistic_spin_queue *lock)
> >   	atomic_set(&lock->tail, OSQ_UNLOCKED_VAL);
> >   }
> >
> > +static inline bool osq_has_spinner(struct optimistic_spin_queue *lock)
> > +{
> > +	return atomic_read(&lock->tail) != OSQ_UNLOCKED_VAL;
> > +}
> Like with other locks, should we make this "osq_is_locked"? We can still
> add the rwsem has_spinner() abstractions which makes use of
> osq_is_locked() if we want.

Yes, that is a good idea. I will make the change in the next version.

-Longman
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