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

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Message-ID: <4BCC3AC2.8050601@redhat.com>
Date:	Mon, 19 Apr 2010 14:13:06 +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 01:56 PM, Peter Zijlstra wrote:
>
>    
>>> Right, do bear in mind that the x86 implementation of atomic64_read() is
>>> terrifyingly expensive, it is better to not do that read and simply use
>>> the result of the cmpxchg.
>>>
>>>
>>>        
>> atomic64_read() _is_ cmpxchg64b.  Are you thinking of some clever
>> implementation for smp i386?
>>      
>
> No, what I was suggesting was to rewrite that loop no to need the
> initial read but use the cmpxchg result of the previous iteration.
>
> Something like:
>
>    u64 last = 0;
>
>    /* more stuff */
>
>    do {
>      if (ret<  last)
>        return last;
>      last = cmpxchg64(&last_value, last, ret);
>    } while (last != ret);
>
> That only has a single cmpxchg8 in there per loop instead of two
> (avoiding the atomic64_read() one).
>    

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.

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

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