Message-ID: <1401915420.13877.20.camel@buesod1.americas.hpqcorp.net>
Subject: Re: [RFC PATCH 1/3] locking/mutex: Try to acquire mutex only if it
 is unlocked
From: Davidlohr Bueso <davidlohr@hp.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Jason Low <jason.low2@hp.com>, mingo@kernel.org, tglx@linutronix.de,
        akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        paulmck@linux.vnet.ibm.com, tim.c.chen@linux.intel.com,
        peter@hurleysoftware.com, riel@redhat.com, hpa@zytor.com,
        walken@google.com, Waiman.Long@hp.com, aswin@hp.com,
        scott.norton@hp.com, chegu_vinod@hp.com
Date: Wed, 04 Jun 2014 13:57:00 -0700
In-Reply-To: <20140604194322.GN13930@laptop.programming.kicks-ass.net>
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	 <1401908911-8947-2-git-send-email-jason.low2@hp.com>
	 <20140604194322.GN13930@laptop.programming.kicks-ass.net>
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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@hp.com>
> > ---
> >  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?

Agreed. 

In addition, how about the following helpers instead:
- mutex_is_unlocked() : count > 0
- mutex_has_waiters() : count < 0, or list_empty(->wait_list)

If combined with the negation, I think that pretty much covers the whole
life-cycle of a mutex. The mutex.c file is full of places where we can
replace crude atomic_reads with these.

Thanks,
Davidlohr

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