Re: [PATCH v4 1/2] locking/rwsem: reduce spinlock contention in wakeup after up_read/up_write

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Apr 30, 2015 at 05:12:16PM -0400, Waiman Long wrote:
> In up_write()/up_read(), rwsem_wake() will be called whenever it
> detects that some writers/readers are waiting. The rwsem_wake()
> function will take the wait_lock and call __rwsem_do_wake() to do the
> real wakeup.  For a heavily contended rwsem, doing a spin_lock() on
> wait_lock will cause further contention on the heavily contended rwsem
> cacheline resulting in delay in the completion of the up_read/up_write
> operations.
> 
> This patch makes the wait_lock taking and the call to __rwsem_do_wake()
> optional if at least one spinning writer is present. The spinning
> writer will be able to take the rwsem and call rwsem_wake() later
> when it calls up_write(). With the presence of a spinning writer,
> rwsem_wake() will now try to acquire the lock using trylock. If that
> fails, it will just quit.
> 
> Signed-off-by: Waiman Long <Waiman.Long@hp.com>
> Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---

Thanks!