Subject: Re: [patch 3/4] net: Percpufy frequently used variables -- proto.sockets_allocated

From: Ravikiran G Thirumalai <[email-masked]>
Date: Fri, 27 Jan 2006 20:52:48 -0800

On Sat, Jan 28, 2006 at 01:35:03AM +0100, Eric Dumazet wrote:
> Eric Dumazet a écrit :
> >Andrew Morton a écrit :
> >>Eric Dumazet <[email-masked]> 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