Re: [RFC PATCH 1/3] locking/mutex: Try to acquire mutex only if it is unlocked

[Waiman Long]

On 06/04/2014 05:26 PM, Jason Low wrote:
> On Wed, 2014-06-04 at 21:43 +0200, Peter Zijlstra wrote:
> > On Wed, Jun 04, 2014 at 12:08:29PM -0700, Jason Low wrote:
> > > Upon entering the slowpath in __mutex_lock_common(), we try once more
> > > to acquire the mutex. We only try to acquire it if MUTEX_SHOW_NO_WAITER
> > > (lock->count>= 0) is true in order to avoid using the atomic xchg()
> > > operation whenever it is not necessary. However, we really only need
> > > to try to acquire if the mutex is free (lock->count == 1).
> > >
> > > This patch changes it so that we only try-acquire the mutex upon
> > > entering the slowpath if it is unlocked, rather than if there are
> > > no waiters. This helps further reduce unncessary atomic xchg()
> > > operations. Furthermore, this patch introduces and uses a new
> > > MUTEX_IS_UNLOCKED() macro to improve readbability.
> > >
> > > Signed-off-by: Jason Low<jason.low2@xxxxxx>
> > > ---
> > >   kernel/locking/mutex.c |   10 ++++++----
> > >   1 files changed, 6 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> > > index bc73d33..0925968 100644
> > > --- a/kernel/locking/mutex.c
> > > +++ b/kernel/locking/mutex.c
> > > @@ -48,9 +48,10 @@
> > >
> > >   /*
> > >    * A negative mutex count indicates that waiters are sleeping waiting for the
> > > - * mutex.
> > > + * mutex, and a count of one indicates the mutex is unlocked.
> > >    */
> > >   #define	MUTEX_SHOW_NO_WAITER(mutex)	(atomic_read(&(mutex)->count)>= 0)
> > > +#define	MUTEX_IS_UNLOCKED(mutex)	(atomic_read(&(mutex)->count) == 1)
> > So I recently saw that MUTEX_SHOW_NO_WAITER thing and cried a little;
> > and now you're adding more of that same nonsense.
> >
> > Please make them inline functions, also can we rename the SHOW_NO_WAITER
> > thing, because its not at all clear to me wtf it does; should it be
> > called: mutex_no_waiters() or somesuch?
> Okay, I can make them inline functions. I mainly added the macro to keep
> it consistent with the MUTEX_SHOW_NO_WAITER() check, but we can surely
> make this more clear. mutex_no_waiters() sounds fine, or perhaps
> something like mutex_has_no_waiters()?

You can remove the MUTEX_SHOW_NO_WAITER macro as all the call sites are 
to be replaced. I didn't check directly for unlocked count because of 
fairness concern in my original patch, but I think checking directly for 
unlocked count should be fine too.

-Longman
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