linux-kernel - Re: [RFC PATCH 3/3] locking/mutex: Optimize mutex trylock slowpath

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Date:	Wed, 04 Jun 2014 14:47:11 -0700
From:	Jason Low <jason.low2@...com>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	mingo@...nel.org, peterz@...radead.org, tglx@...utronix.de,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	paulmck@...ux.vnet.ibm.com, tim.c.chen@...ux.intel.com,
	peter@...leysoftware.com, riel@...hat.com, hpa@...or.com,
	walken@...gle.com, Waiman.Long@...com, aswin@...com,
	scott.norton@...com, chegu_vinod@...com
Subject: Re: [RFC PATCH 3/3] locking/mutex: Optimize mutex trylock slowpath

On Wed, 2014-06-04 at 13:28 -0700, Davidlohr Bueso wrote:
> On Wed, 2014-06-04 at 12:08 -0700, Jason Low wrote:
> > In __mutex_trylock_slowpath(), we acquire the wait_lock spinlock,
> > xchg() lock->count with -1, then set lock->count back to 0 if there
> > are no waiters, and return true if the prev lock count was 1.
> > 
> > However, if we the mutex is already locked, then there may not be
>              ^^ leave that out.
> 
> > much point in attempting the above operations. 
> 
> Isn't this redundant? I mean, if we enter the slowpath its because
> __mutex_fastpath_trylock() already failed so we already know that the
> lock is taken.

This function is really just used as an alternative method of trylock
for !__HAVE_ARCH_CMPXCHG.  In that case, the fastpath can call directly
into the slowpath function, without checking for if the lock is taken.

> What kind of testing has this change been put through? Any advantages?
> (ie: how many cycles are we saving here?), the trylock mechanism is
> already pretty darn fast.

While I did run tests with this patch, this particular patch shouldn't
show benefits on my machine as it should be using the more efficient
atomic_cmpxchg. The advantage is in !__HAVE_ARCH_CMPXCHG, where we would
avoid taking a spinlock and 2 atomic operations when the mutex is
already taken.

Thanks.

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