Re: light weight counters: race free through local_t?

Zoltan Menyhart
2006-06-16T09:14:52Z

Let's put it in another way:
Do the statistics need to be absolutely precise?
I guess they do not.
By the time you can display them, they have already been changed.

Let's take an example:

zone_statistics(struct zonelist *zonelist, struct zone *z)
{
...
// local_irq_save(flags); // No IRQ lock out
cpu = smp_processor_id(); // Can become another CPU
p = &z->pageset[cpu]; // Can count for someone else
if (pg = orig) {
stat_incr(&z->pageset[cpu].numa_hit); // Unsafe
} else {
...
// local_irq_restore(flags);
}

Where "stat_incr()" is arch. dependent and possibly unsafe routine.

For IA64:

// Unsafe statistics
static inline void stat_incr(int *addr){
int tmp;

// Obtain immediately the cache line exclusivity, do not touch L1
asm volatile ("ld4.bias.nt1 %0=[%1]" : "=r"(tmp) : "r" (addr));
tmp++;
asm volatile ("st4 [%1] = %0" :: "r"(tmp), "r"(addr) : "memory");
}

It takes 10 clock cycles.

Regards,

Zoltan