On Sat, Jan 28, 2006 at 01:35:03AM +0100, Eric Dumazet wrote:
> Eric Dumazet a écrit :
> >Andrew Morton a écrit :
> >>Eric Dumazet <[email protected]> wrote:
> >
> >#ifdef CONFIG_SMP
> >void percpu_counter_mod(struct percpu_counter *fbc, long amount)
> >{
> > long old, new;
> > atomic_long_t *pcount;
> >
> > pcount = per_cpu_ptr(fbc->counters, get_cpu());
> >start:
> > old = atomic_long_read(pcount);
> > new = old + amount;
> > if (new >= FBC_BATCH || new <= -FBC_BATCH) {
> > if (unlikely(atomic_long_cmpxchg(pcount, old, 0) != old))
> > goto start;
> > atomic_long_add(new, &fbc->count);
> > } else
> > atomic_long_add(amount, pcount);
> >
> > put_cpu();
> >}
> >EXPORT_SYMBOL(percpu_counter_mod);
> >
> >long percpu_counter_read_accurate(struct percpu_counter *fbc)
> >{
> > long res = 0;
> > int cpu;
> > atomic_long_t *pcount;
> >
> > for_each_cpu(cpu) {
> > pcount = per_cpu_ptr(fbc->counters, cpu);
> > /* dont dirty cache line if not necessary */
> > if (atomic_long_read(pcount))
> > res += atomic_long_xchg(pcount, 0);
---------------------------> (A)
> > }
>
> atomic_long_add(res, &fbc->count);
---------------------------> (B)
> res = atomic_long_read(&fbc->count);
>
> > return res;
> >}
The read is still theoritically FBC_BATCH * NR_CPUS inaccurate no?
What happens when other cpus update their local counters at (A) and (B)?
(I am hoping we don't need percpu_counter_read_accurate anywhere yet and
this is just demo ;). I certainly don't want to go on all cpus for read /
add cmpxchg on the write path for the proto counters that started this
discussion)
Thanks,
Kiran
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