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    SubjectRe: [PATCH 1/5] Add a global synchronization point for pvclock
    On Mon, Apr 19, 2010 at 01:19:43PM +0200, Peter Zijlstra wrote:
    > On Mon, 2010-04-19 at 14:13 +0300, Avi Kivity wrote:
    > > 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.
    >
    > 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);
    isn't a barrier enough here?




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