Re: [PATCH] percpu_counter: Fix __percpu_counter_sum()

Eric Dumazet
2008-12-12T08:23:48Z

Rusty Russell a écrit :
> On Thursday 11 December 2008 09:26:37 Eric Dumazet wrote:
>> But then, some (all but x86 ;) ) arches dont have true local_t and we fallback
>> to plain atomic_long_t, and this is wrong because it would add a LOCKED
>> instruction in fast path.
>>
>> I remember Christoph added FAST_CMPXCHG_LOCAL, but no more uses of it in current
>> tree.
>>
>> Ie : using local_t only if CONFIG_FAST_CMPXCHG_LOCAL, else something like :
>>
>> void __percpu_counter_add_irqsafe(struct percpu_counter *fbc, s64 amount, s32 batch)
>> {
>> s64 count;
>> s32 *pcount = per_cpu_ptr(fbc->counters, get_cpu());
>> unsigned long flags;
>>
>> local_irq_save(flags);
>> count = *pcount + amount;
>
> This is dumb though. If local_irq_save(), add, local_irq_restore() is faster
> than atomic_long_add on some arch, *that* is what that arch's local_add()
> should do!
>
> Open coding it like this is obviously wrong.

Hum... so you vote for using local_t instead of s32 then ?

>
> Now, archs local.h need attention (x86-32 can be optimized today, for
> example), but that's not directly related.
>
> Hope that clarifies,
> Rusty.
> PS. Yes, I should produce a documentation patch and fix the x86 version.
> Added to TODO list.
>

Thanks