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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