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

[Waiman Long]

On 08/04/2014 12:10 AM, Jason Low wrote:
> 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@xxxxxx>
> > ---
> >   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.

This patch set enables proper reader spinning and so the problem that we 
see with xfs_repair workload should go away. I should have this patch 
after patch 4 to make it less confusing. BTW, patch 3 can significantly 
reduce spinlock contention in rwsem. So I believe the xfs_repair 
workload should run faster with this patch than both 3.15 and 3.16.

-Longman
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