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SubjectRe: [rfc][patch] queued spinlocks (i386)
On Fri, 23 Mar 2007 09:59:11 +0100
Nick Piggin <npiggin@suse.de> wrote:

>
> Implement queued spinlocks for i386. This shouldn't increase the size of
> the spinlock structure, while still able to handle 2^16 CPUs.
>
> Not completely implemented with assembly yet, to make the algorithm a bit
> clearer.
>
> The queued spinlock has 2 fields, a head and a tail, which are indexes
> into a FIFO of waiting CPUs. To take a spinlock, a CPU performs an
> "atomic_inc_return" on the head index, and keeps the returned value as
> a ticket. The CPU then spins until the tail index is equal to that
> ticket.
>
> To unlock a spinlock, the tail index is incremented (this can be non
> atomic, because only the lock owner will modify tail).
>
> Implementation inefficiencies aside, this change should have little
> effect on performance for uncontended locks, but will have quite a
> large cost for highly contended locks [O(N) cacheline transfers vs
> O(1) per lock aquisition, where N is the number of CPUs contending].
> The benefit is is that contended locks will not cause any starvation.
>
> Just an idea. Big NUMA hardware seems to have fairness logic that
> prevents starvation for the regular spinlock logic. But it might be
> interesting for -rt kernel or systems with starvation issues.

It's a very nice idea Nick.

You also have for free the number or cpus that are before you.

On big SMP/NUMA, we could use this information to call a special lock_cpu_relax() function to avoid cacheline transferts.

asm volatile(LOCK_PREFIX "xaddw %0, %1\n\t"
: "+r" (pos), "+m" (lock->qhead) : : "memory");
for (;;) {
unsigned short nwait = pos - lock->qtail;
if (likely(nwait == 0))
break;
lock_cpu_relax(lock, nwait);
}
lock_cpu_relax(raw_spinlock_t *lock, unsigned int nwait)
{
unsigned int cycles = nwait * lock->min_cycles_per_round;
busy_loop(cycles);
}
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