linux-kernel - Re: [PATCH 1/5] Add a global synchronization point for pvclock

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Message-ID: <4BCC4111.4030405@redhat.com>
Date:	Mon, 19 Apr 2010 14:40:01 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Glauber Costa <glommer@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>
Subject: Re: [PATCH 1/5] Add a global synchronization point for pvclock

On 04/19/2010 02:19 PM, Peter Zijlstra wrote:
>
>> Still have two cmpxchgs in the common case.  The first iteration will
>> fail, fetching last_value, the second will work.
>>
>> It will be better when we have contention, though, so it's worthwhile.
>>      
> Right, another option is to put the initial read outside of the loop,
> that way you'll have the best of all cases, a single LOCK'ed op in the
> loop, and only a single LOCK'ed op for the fast path on sensible
> architectures ;-)
>
>      last = atomic64_read(&last_value);
>      do {
>        if (ret<  last)
>          return last;
>        last = atomic64_cmpxchg(&last_value, last, ret);
>      } while (unlikely(last != ret));
>
> Or so.
>
>    

Yes, looks optimal when !NONSTOP_TSC.

-- 
error compiling committee.c: too many arguments to function

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