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
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