Message-ID: <1430243958.4463.10.camel@j-VirtualBox>
Subject: Re: [PATCH v3] locking/rwsem: reduce spinlock contention in wakeup
 after up_read/up_write
From: Jason Low <jason.low2@hp.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Waiman Long <Waiman.Long@hp.com>, Ingo Molnar <mingo@kernel.org>,
        linux-kernel@vger.kernel.org, Davidlohr Bueso <dave@stgolabs.net>,
        Scott J Norton <scott.norton@hp.com>,
        Douglas Hatch <doug.hatch@hp.com>, jason.low2@hp.com
Date: Tue, 28 Apr 2015 10:59:18 -0700
In-Reply-To: <1430243401.4463.4.camel@j-VirtualBox>
References: <1429898069-28907-1-git-send-email-Waiman.Long@hp.com>
	 <20150428171734.GH23123@twins.programming.kicks-ass.net>
	 <1430243401.4463.4.camel@j-VirtualBox>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 7bit
Mime-Version: 1.0
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1673
Lines: 74

On Tue, 2015-04-28 at 10:50 -0700, Jason Low wrote:
> On Tue, 2015-04-28 at 19:17 +0200, Peter Zijlstra wrote:
> 
> > To me it makes more sense to reverse these two branches (identical code
> > wise of course) and put the special case first.
> > 
> > Alternatively we could also do something like the below, which to my
> > eyes looks a little better still, but I don't care too much.
> > 
> > 	if (rwsem_has_spinner(sem)) {
> > 		/*
> > 		 * comment ...
> > 		 */
> > 		 smp_rmb();
> > 		 if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
> > 			return sem;
> > 		 goto locked;
> > 	}
> > 
> > 	raw_spin_lock_irqsave(&sem->wait_lock, flags);
> > locked:
> 
> How about putting this into its own function:
> 
> static inline bool __rwsem_wake_acquire_wait_lock(sem)
> {
> 	/*
> 	 *
> 	 * Comments
> 	 *
> 	 */
> 	if (unlikely(rwsem_has_spinner(sem))) {
> 		/*
> 		 * Comments
> 		 */
> 		smp_rmb();
> 		if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
> 			return false;
> 	}
> 
> 	return true;
> }

That is, with the raw_spin_lock_irqsave() too :)

static inline bool __rwsem_wake_acquire_wait_lock(sem)
{
 	/*
 	 *
 	 * Comments
 	 *
 	 */
 	if (unlikely(rwsem_has_spinner(sem))) {
 		/*
 		 * Comments
 		 */
 		smp_rmb();
 		if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
 			return false;

		/* trylock successful */
		return true;
 	}

	raw_spin_lock_irqsave(&sem->wait_lock, flags);
 	return true;
}


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/