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

From: Eric Dumazet <[email-masked]>
Date: Sat, 28 Jan 2006 01:35:03 +0100

Eric Dumazet a écrit :
> Andrew Morton a écrit :
>> Eric Dumazet <[email-masked]> wrote:
>>> Ravikiran G Thirumalai a écrit :
>>>> On Fri, Jan 27, 2006 at 12:16:02PM -0800, Andrew Morton wrote:
>>>>> Ravikiran G Thirumalai <[email-masked]> wrote:
>>>>>> which can be assumed as not frequent.  At 
>>>>>> sk_stream_mem_schedule(), read_sockets_allocated() is invoked only 
>>>>>> certain conditions, under memory pressure -- on a large CPU count 
>>>>>> machine, you'd have large memory, and I don't think 
>>>>>> read_sockets_allocated would get called often.  It did not atleast 
>>>>>> on our 8cpu/16G box.  So this should be OK I think.
>>>>> That being said, the percpu_counters aren't a terribly successful 
>>>>> concept
>>>>> and probably do need a revisit due to the high inaccuracy at high CPU
>>>>> counts.  It might be better to do some generic version of 
>>>>> vm_acct_memory()
>>>>> instead.
>>>> AFAICS vm_acct_memory is no better.  The deviation on large cpu 
>>>> counts is the same as percpu_counters -- (NR_CPUS * NR_CPUS * 2) ...
>>> Ah... yes you are right, I read min(16, NR_CPUS*2)
>>
>> So did I ;)
>>
>>> I wonder if it is not a typo... I mean, I understand the more cpus 
>>> you have, the less updates on central atomic_t is desirable, but a 
>>> quadratic offset seems too much...
>>
>> I'm not sure whether it was a mistake or if I intended it and didn't 
>> do the
>> sums on accuracy :(
>>
>> An advantage of retaining a spinlock in percpu_counter is that if 
>> accuracy
>> is needed at a low rate (say, /proc reading) we can take the lock and 
>> then
>> go spill each CPU's local count into the main one.  It would need to be a
>> very low rate though.   Or we make the cpu-local counters atomic too.
> 
> We might use atomic_long_t only (and no spinlocks)
> Something like this ?
> 
> 
> ------------------------------------------------------------------------
> 
> 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 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);
> 	}

	atomic_long_add(res, &fbc->count);
	res = atomic_long_read(&fbc->count);

> 	return res;
> }
> EXPORT_SYMBOL(percpu_counter_read_accurate);
> #endif /* CONFIG_SMP */
>