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

Avi Kivity
2010-04-19T11:13:06Z

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