Re: 2.4.23aa2 (bugfixes and important VM improvements for the high end)

[Jamie Lokier <jamie@shareable.org>]

Andrea Arcangeli wrote:
> > Can you use a read-write lock, so that userspace copies only need to
> > take the lock for reading?  That doesn't eliminate cacheline bouncing
> > but does eliminate the serialisation.
> 
> normally the bouncing would be the only overhead, but here I also think
> the serialization is a significant factor of the contention because the
> critical section is taking lots of time. So I would expect some
> improvement by using a read/write lock.

For something as significant as user<->kernel data transfers, it might
be worth eliminating the bouncing as well - by using per-CPU * per-mm
spinlocks.

User<->kernel data transfers would take the appropriate per-CPU lock
for the current mm, and not take page_table_lock.  Everything that
normally takes page_table_lock would, and also take all of the per-CPU locks.

That does require a set of per-CPU spinlocks to be allocated whenever
a new mm is allocated (although the sets could be cached so it needn't
be slow).

-- Jamie