Ravikiran G Thirumalai a écrit :
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:
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)?
Well, after atomic_read(&some_atomic) or percpu_counter_read_accurate(&s) the
'value' you got may be inaccurate by whatever... You cannot 'freeze the
atomic / percpu_counter and gets its accurate value'. All you can do is
'atomically add/remove some value from this counter (atomic or percpu_counter))
See percpu_counter_read_accurate() as an attempt to collect all local offset
(and atomically set them to 0) and atomically reajust the central fbc->count
to with the sum of all these offsets. Kind of a consolidation that might be
driven by a user request (read /proc/sys/...) or a periodic timer.
(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)
As pointed by Andrew (If you decode carefully its short sentences :) ), all
you really need is atomic_long_xchg() (only in slow path), and
atomic_long_{read/add} in fast path)
Eric
struct percpu_counter {
atomic_long_t count;
atomic_long_t *counters;
};
#ifdef CONFIG_SMP
void percpu_counter_mod(struct percpu_counter *fbc, long amount)
{
long new;
atomic_long_t *pcount;
pcount = per_cpu_ptr(fbc->counters, get_cpu());
new = atomic_long_read(pcount) + amount;
if (new >= FBC_BATCH || new <= -FBC_BATCH) {
new = atomic_long_xchg(pcount, 0) + amount;
if (new)
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);
}
atomic_long_add(res, &fbc->count);
return atomic_long_read(&fbc->count);
}
EXPORT_SYMBOL(percpu_counter_read_accurate);
#endif /* CONFIG_SMP */
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