On Fri, 2010-09-17 at 16:22 +0200, Andi Kleen wrote:
> > I don't disagree with the idea of removing it, especially as it has so
> > few users, but replacing the host lock with an atomic here would still
> > vastly reduce the contention, which is the initial complaint. The
>
> Actually the complaint is the overhead of the spin lock. This can be
> either caused
> by contention or by cache line bounce time.
The original complaint was contention. My desire is to reduce the
locked path coverage, so I saw an opportunity.
What I was actually thinking of for the atomic is that we'd let it range
[1..INT_MAX] so a zero was an indicator of no use of this. Then the
actual code could become
if (atomic_read(x)) {
do {
y = atomic_add_return(1, x);
} while (y == 0);
}
So "fast" cards not using the serial number set a zero there (we'd
default initialise to one), the line is shared so no bouncing (because
it's never updated). This should satisfy everyone.
> > contention occurs because the host lock is so widely used for other
> > things. The way to reduce that contention is firstly to reduce the
> > length of code covered by the lock and also reduce the actual number of
> > places where the lock is taken. Compared with host lock's current vast
> > footprint, and atomic here is tiny.
>
> That assumes that it's contention that is the problem and not simply
> bounce time.
That's what the patch and data that started this whole thread showed,
yes ... but I think actual bounce in the spinlock is also a problem ...
we just don't have data to show it.
James
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