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

[Jason Low <jason.low2-VXdhtT5mjnY@public.gmane.org>]

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-VXdhtT5mjnY@public.gmane.org>
> ---
>  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.