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SubjectRe: [RFC][PATCH 6/8] mm: handle_speculative_fault()

On Tue, 5 Jan 2010, Paul E. McKenney wrote:
> But on many systems, it does take some time for the idle reads to make
> their way to the CPU that just acquired the lock.

Yes. But the point is that there is lots of them.

So think of it this way: every time _one_ CPU acquires a lock (and
then releases it), _all_ CPU's will read the new value. Imagine the
cross-socket traffic.

In contrast, doing just a single xadd (which replaces the whole
"spin_lock+non-atomics+spin_unlock"), every times _once_ CPU cquires a
lock, that's it. The other CPU's arent' all waiting in line for the lock
to be released, and reading the cacheline to see if it's their turn.

Sure, after they got the lock they'll all eventually end up reading from
that cacheline that contains 'struct mm_struct', but that's something we
could even think about trying to minimize by putting the mmap_sem as far
away from the other fields as possible.

Now, it's very possible that if you have a broadcast model of cache
coherency, none of this much matters and you end up with almost all the
same bus traffic anyway. But I do think it could matter a lot.


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