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

Glauber Costa
2010-04-19T14:32:27Z

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?