From: Eric Dumazet <dada1@cosmosbay.com>
To: Ravikiran G Thirumalai <kiran@scalex86.org>
Cc: Andrew Morton <akpm@osdl.org>,
davem@davemloft.net, linux-kernel@vger.kernel.org,
shai@scalex86.org, netdev@vger.kernel.org,
pravins@calsoftinc.com
Subject: Re: [patch 3/4] net: Percpufy frequently used variables -- proto.sockets_allocated
Date: Sat, 28 Jan 2006 08:19:12 +0100 [thread overview]
Message-ID: <43DB1AF0.8040108@cosmosbay.com> (raw)
In-Reply-To: <20060128045248.GA3584@localhost.localdomain>
[-- Attachment #1: Type: text/plain, Size: 1929 bytes --]
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 <dada1@cosmosbay.com> 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
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[-- Type: text/plain, Size: 1050 bytes --]
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 */
next prev parent reply other threads:[~2006-01-28 7:19 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-01-26 18:56 [patch 0/4] net: Percpufy frequently used variables on struct proto Ravikiran G Thirumalai
2006-01-26 18:59 ` [patch 1/4] net: Percpufy frequently used variables -- add percpu_counter_mod_bh Ravikiran G Thirumalai
2006-01-26 19:02 ` [patch 2/4] net: Percpufy frequently used variables -- struct proto.memory_allocated Ravikiran G Thirumalai
2006-01-27 9:01 ` Eric Dumazet
2006-01-26 19:03 ` [patch 3/4] net: Percpufy frequently used variables -- proto.sockets_allocated Ravikiran G Thirumalai
2006-01-27 8:53 ` Eric Dumazet
2006-01-27 19:52 ` Ravikiran G Thirumalai
2006-01-27 20:16 ` Andrew Morton
2006-01-27 22:30 ` Eric Dumazet
2006-01-27 22:50 ` Ravikiran G Thirumalai
2006-01-27 23:21 ` Eric Dumazet
2006-01-28 0:40 ` Ravikiran G Thirumalai
2006-01-27 22:44 ` Ravikiran G Thirumalai
2006-01-27 22:58 ` Eric Dumazet
2006-01-27 23:16 ` Andrew Morton
2006-01-28 0:28 ` Eric Dumazet
2006-01-28 0:35 ` Eric Dumazet
2006-01-28 4:52 ` Ravikiran G Thirumalai
2006-01-28 7:19 ` Eric Dumazet [this message]
2006-01-28 0:43 ` Andrew Morton
2006-01-28 1:10 ` Eric Dumazet
2006-01-28 1:18 ` Andrew Morton
2006-01-29 0:44 ` Benjamin LaHaise
2006-01-29 0:55 ` Andrew Morton
2006-01-29 1:19 ` Benjamin LaHaise
2006-01-29 1:29 ` Andrew Morton
2006-01-29 1:45 ` Kyle McMartin
2006-01-29 5:38 ` Andi Kleen
2006-01-29 6:54 ` Eric Dumazet
2006-01-29 19:52 ` Benjamin LaHaise
2006-01-27 23:01 ` Andrew Morton
2006-01-27 23:08 ` Andrew Morton
2006-01-28 0:01 ` Ravikiran G Thirumalai
2006-01-28 0:26 ` Andrew Morton
2006-02-03 3:05 ` Ravikiran G Thirumalai
2006-02-03 3:16 ` Andrew Morton
2006-02-03 19:37 ` Ravikiran G Thirumalai
2006-02-03 20:13 ` Andrew Morton
2006-01-26 19:05 ` [patch 4/4] net: Percpufy frequently used variables -- proto.inuse Ravikiran G Thirumalai
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