linux-kernel - Re: [PATCH] locking/rwsem: Optimize write lock slowpath

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Message-ID: <1462992562.2488.26.camel@j-VirtualBox>
Date:	Wed, 11 May 2016 11:49:22 -0700
From:	Jason Low <jason.low2@...com>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Scott J Norton <scott.norton@....com>,
	Waiman Long <Waiman.Long@....com>, peter@...leysoftware.com,
	jason.low2@....com
Subject: Re: [PATCH] locking/rwsem: Optimize write lock slowpath

On Wed, 2016-05-11 at 11:33 -0700, Davidlohr Bueso wrote:
> On Wed, 11 May 2016, Peter Zijlstra wrote:
> 
> >On Mon, May 09, 2016 at 12:16:37PM -0700, Jason Low wrote:
> >> When acquiring the rwsem write lock in the slowpath, we first try
> >> to set count to RWSEM_WAITING_BIAS. When that is successful,
> >> we then atomically add the RWSEM_WAITING_BIAS in cases where
> >> there are other tasks on the wait list. This causes write lock
> >> operations to often issue multiple atomic operations.
> >>
> >> We can instead make the list_is_singular() check first, and then
> >> set the count accordingly, so that we issue at most 1 atomic
> >> operation when acquiring the write lock and reduce unnecessary
> >> cacheline contention.
> >>
> >> Signed-off-by: Jason Low <jason.low2@...com>
> 
> Acked-by: Davidlohr Bueso <dave@...olabs.net>
> 
> (one nit: the patch title could be more informative to what
> optimization we are talking about here... ie: reduce atomic ops
> in writer slowpath' or something.)

Yeah, the "optimize write lock slowpath" subject is a bit generic. I'll
make the title more specific in the next version.

Thanks,
Jason