Subject: Re: [RFC] percpu: convert SNMP mibs to new infra

From: Eric Dumazet <[email-masked]>
Date: Thu, 02 Apr 2009 07:19:19 +0200

Rusty Russell a écrit :
> On Thursday 02 April 2009 05:14:47 Eric Dumazet wrote:
>> Here is a preliminary patch for SNMP mibs that seems to work well on x86_32
>>
>> [RFC] percpu: convert SNMP mibs to new infra
> 
> OK, I have a whole heap of "convert to dynamic per-cpu" patches waiting in
> the wings too, once Tejun's conversion is complete.
> 
> Also, what is optimal depends on the arch: we had a long discussion on this
> (it's what local_t was supposed to do, with cpu_local_inc() etc: see
> Subject: local_add_return 2008-12-16 thread).
> 
> eg. on S/390, atomic_inc is a win over the two-counter version.  On Sparc,
> two-counter wins.  On x86, inc wins (obviously).
> 
> But efforts to create a single primitive have been problematic: maybe
> open-coding it like this is the Right Thing.
> 

I tried to find a generic CONFIG_ define that would annonce that an arche
has a fast percpu_add() implementation. (faster than __raw_get_cpu_var,
for example, when we already are in a preempt disabled section)

Any idea ?

For example, net/ipv4/route.c has :

static DEFINE_PER_CPU(struct rt_cache_stat, rt_cache_stat);
#define RT_CACHE_STAT_INC(field) \
        (__raw_get_cpu_var(rt_cache_stat).field++)

We could use percpu_add(rt_cache_stat.field, 1) instead, only if percpu_add()
is not the generic one.

#define __percpu_generic_to_op(var, val, op)                            \
do {                                                                    \
        get_cpu_var(var) op val;                                        \
        put_cpu_var(var);                                               \
} while (0)
#ifndef percpu_add
# define percpu_add(var, val)           __percpu_generic_to_op(var, (val), +=)
#endif