Re: [PATCH 2/7] locking/rwsem: more aggressive use of optimistic spinning

[Jason Low <jason.low2@hp.com>]

On Sun, 2014-08-03 at 22:36 -0400, Waiman Long wrote:
> The rwsem_can_spin_on_owner() function currently allows optimistic
> spinning only if the owner field is defined and is running. That is
> too conservative as it will cause some tasks to miss the opportunity
> of doing spinning in case the owner hasn't been able to set the owner
> field in time or the lock has just become available.
> 
> This patch enables more aggressive use of optimistic spinning by
> assuming that the lock is spinnable unless proved otherwise.
> 
> Signed-off-by: Waiman Long <Waiman.Long@hp.com>
> ---
>  kernel/locking/rwsem-xadd.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index d058946..dce22b8 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -285,7 +285,7 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
>  static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
>  {
>  	struct task_struct *owner;
> -	bool on_cpu = false;
> +	bool on_cpu = true;	/* Assume spinnable unless proved not to be */

Hi,

So "on_cpu = true" was recently converted to "on_cpu = false" in order
to address issues such as a 5x performance regression in the xfs_repair
workload that was caused by the original rwsem optimistic spinning code.

However, patch 4 in this patchset does address some of the problems with
spinning when there are readers. CC'ing Dave Chinner, who did the
testing with the xfs_repair workload.