linux-kernel - Re: [PATCH v2 0/7] locking/rwsem: enable reader opt-spinning & writer respin

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Date:	Fri, 08 Aug 2014 14:16:10 -0400
From:	Waiman Long <waiman.long@...com>
To:	Davidlohr Bueso <davidlohr@...com>
CC:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Jason Low <jason.low2@...com>,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH v2 0/7] locking/rwsem: enable reader opt-spinning & writer
 respin

On 08/07/2014 07:52 PM, Davidlohr Bueso wrote:
> On Thu, 2014-08-07 at 18:26 -0400, Waiman Long wrote:
>> v1->v2:
>>   - Remove patch 1 which changes preempt_enable() to
>>     preempt_enable_no_resched().
>>   - Remove the RWSEM_READ_OWNED macro and assume readers own the lock
>>     when owner is NULL.
>>   - Reduce the spin threshold to 64.
> So I still don't like this, and the fact that it is used in some
> virtualization locking bits doesn't really address the concerns about
> arbitrary logic in our general locking code.

As I said in the comments, there is no easy way to figure if all the 
readers are running. I set the spin count to a relatively small number 
to catch those readers with a short critical sections. For those that 
hold the lock for a relatively long time, the spin will end and the task 
will be put to sleep. I know the solution is not elegant, but it is 
simple. I thought about using more elaborate scheme, but there is no 
guarantee that that it will be better than a simple spin count while 
greatly complicating the code.

> Also, why did you reduce it from 100 to 64? This very much wants to be
> commented.

In the v1 patch, the 100 spin threshold was for the whole spinning 
period. In the v2 patch, I reset the count when a writer is there. There 
is why I reduce the spin count a bit.

-Longman
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