Re: [PATCH v4 5/5] rwsem: do optimistic spinning for writer lockacquisition

[Tim Chen]

On Wed, 2013-06-26 at 15:40 -0700, Davidlohr Bueso wrote:
> On Wed, 2013-06-26 at 15:28 -0700, Tim Chen wrote:
> > We want to add optimistic spinning to rwsems because we've noticed that
> > the writer rwsem does not perform as well as mutexes. Tim noticed that
> > for exim (mail server) workloads, when reverting commit 4fc3f1d6 and Davidlohr
> > noticed it when converting the i_mmap_mutex to a rwsem in some aim7
> > workloads. We've noticed that the biggest difference, in a nutshell, is
> > when we fail to acquire a mutex in the fastpath, optimistic spinning
> > comes in to play and we can avoid a large amount of unnecessary sleeping
> > and wait queue overhead.
> > 
> > For rwsems on the other hand, upon entering the writer slowpath in
> > rwsem_down_write_failed(), we just acquire the ->wait_lock, add
> > ourselves to the wait_queue and blocking until we get the lock.
> > 
> > Reviewed-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Reviewed-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > Signed-off-by: Davidlohr Bueso <davidlohr.bueso@xxxxxx>
> > ---
> >  include/linux/rwsem.h |    3 +
> >  init/Kconfig          |    9 +++
> >  kernel/rwsem.c        |   29 +++++++++-
> >  lib/rwsem.c           |  150 +++++++++++++++++++++++++++++++++++++++++++++----
> >  4 files changed, 179 insertions(+), 12 deletions(-)
> > 
> > diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
> > index 0616ffe..0c5933b 100644
> > --- a/include/linux/rwsem.h
> > +++ b/include/linux/rwsem.h
> > @@ -29,6 +29,9 @@ struct rw_semaphore {
> >  #ifdef CONFIG_DEBUG_LOCK_ALLOC
> >  	struct lockdep_map	dep_map;
> >  #endif
> > +#ifdef CONFIG_RWSEM_SPIN_ON_WRITE_OWNER
> > +	struct task_struct	*owner;
> > +#endif
> >  };
> >  
> >  extern struct rw_semaphore *rwsem_down_read_failed(struct rw_semaphore *sem);
> > diff --git a/init/Kconfig b/init/Kconfig
> > index 9d3a788..1c582d1 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -1595,6 +1595,15 @@ config TRACEPOINTS
> >  
> >  source "arch/Kconfig"
> >  
> > +config RWSEM_SPIN_ON_WRITE_OWNER
> > +	bool "Optimistic spin write acquisition for writer owned rw-sem"
> > +	default n
> > +	depends on SMP
> > +	help
> > +	  Allows a writer to perform optimistic spinning if another writer own
> > +	  the read write semaphore.  This gives a greater chance for writer to
> > +	  acquire a semaphore before blocking it and putting it to sleep.
> > +
> 
> Quoting from kernel/mutex.c:
> 
> "The rationale is that if the lock owner is running, it is likely to
> release the lock soon." 
> 
> It would be good to add that to the Kconfig.

Sounds good.

Tim


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