Re: [PATCH 1/3] netfilter : 3 patches to boost ip_tables performance

Eric Dumazet
2005-09-22T13:17:56Z

Harald Welte a écrit :
> On Wed, Sep 21, 2005 at 11:29:13PM +0200, Eric Dumazet wrote:
>
>>Patch 1/3
>>
>>1) No more one rwlock_t protecting the 'curtain'
>
>
> I have no problem with this change "per se", but with the
> implementation.
>
> As of now, we live without any ugly #ifdef CONFIG_SMP / #endif sections
> in the code - and if possible, I would continue this good tradition.
>
> For example the get_counters() function. Wouldn't all the smp specific
> code (for_each_cpu(), ...) be #defined to nothing anyway?

Well... not exactly, but you are right only the first loop (SET_COUNTER) will
really do something. The if (cpu == curcpu) will be true but the compiler wont
know that, cpu and curcpu are still C variables.

>
> And if we really need the #ifdef's, I would appreciate if those
> sectionas are as small as possible. in get_counters() the section can
> definitely be smaller, rather than basically having the whole function
> body separate for smp and non-smp cases.

get_counters() is not critical, so I agree with you we can stick the general
version (not the UP optimized one)

>
> Also, how much would we loose in runtime performance if we were using a
> "rwlock_t *" even in the UP case?. I mean, it's just one more pointer
> dereference of something that is expected to be in cache anyway, isn't
> it? This gets rid of another huge set of #ifdefs that make the code
> unreadable and prone to errors being introduced later on.
>

Well, in UP case, the rwlock_t is a nulldef.

I was inspired by another use of percpu data in include/linux/genhd.h
#ifdef CONFIG_SMP
struct disk_stats *dkstats;
#else
struct disk_stats dkstats;
#endif

But if you dislike this, we can use pointer for all cases.

Eric