Re: queued spinlock code and results

[Andi Kleen <andi@firstfloor.org>]

Nick Piggin <npiggin@suse.de> writes:

> I made some tests of the queued spinlock code using userspace test code on
> 64-bit processors. I believe the xadd based code no longer has any theoretical
> memory ordering problems.

Linus, the background of this is that on 8 socket Opteron systems
the current spinlocks can become very unfair to the point of severe 
starvation. These boxes are becomming more common.

> The threaded results also attempt to have an unfairness count, which is the
> max number of times in a row that a lock is acquired,  when all other threads
> are also executing in the loop -- the reason xadd for example is not always
> 0 there is because the other threads may not have reached the lock before
> the current thread was able to get it several times (eg. if an interrupt
> comes in, this could happen).

Interesting. I was also thinking about switching the lock types
at boot time. Since all the lock calls are out of line this would
be reasonably easy.

I would say the main drawback of switchable and queued locks 
would be also that they require a larger spinlock_t thus increasing
cache usage

e.g. it would probably hurt for the large spinlock tables used 
by TCP, but then those should be fixed anyways to be smaller.

-Andi