linux-kernel - Re: [RFC PATCH-tip v2 2/6] locking/rwsem: Stop active read lock ASAP

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Date:	Thu, 16 Jun 2016 17:25:35 -0400
From:	Waiman Long <waiman.long@....com>
To:	Davidlohr Bueso <dave@...olabs.net>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	<x86@...nel.org>, <linux-alpha@...r.kernel.org>,
	<linux-ia64@...r.kernel.org>, <linux-s390@...r.kernel.org>,
	<linux-arch@...r.kernel.org>, Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH-tip v2 2/6] locking/rwsem: Stop active read lock ASAP

On 06/15/2016 10:14 PM, Davidlohr Bueso wrote:
> On Wed, 15 Jun 2016, Waiman Long wrote:
>
>> I think there will be a little bit of performance impact for a 
>> workload that produce just the right amount of rwsem contentions. 
>
> I'm not saying the change doesn't make sense, but this is the sort of
> thing that will show nice numbers in one workload and go bite you in
> another.
>
> Thanks,
> Davidlohr

I would certainly agree if the additional atomic op is in the fastpath. 
Since it is in the slowpath, one additional atomic op will just be a 
small part of the whole rwsem_down_read_failed() function. I doubt if 
the performance degradation, if any, can be even noticeable.

Cheers,
Longman