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    SubjectRe: [RFC] percpu: convert SNMP mibs to new infra
    Date
    On Thursday 02 April 2009 15:49:19 Eric Dumazet wrote:
    > Rusty Russell a écrit :
    > > 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)

    Nope, we don't have one. It was supposed to work like this:
    DEFINE_PER_CPU(local_t, counter);

    cpu_local_inc(counter);

    That would do incl in x86, local_t could even be a long[3] (one for hardirq,
    one for softirq, one for user context). But there were issues:

    1) It didn't work on dynamic percpu allocs, which was much of the interesting
    use (Tejun is fixing this bit right now)
    2) The x86 version wasn't optimized anyway,
    3) Everyone did atomic_long_inc(), so the ftrace code assumed it would be nmi
    safe (tho atomic_t isn't nmi-safe on some archs anyway), so the long[3]
    method would break them,
    4) The long[3] version was overkill for networking, which doesn't need hardirq
    so we'd want another variant of local_t plus all the ops,
    5) Some people didn't want long: Christoph had a more generic but more complex
    version,
    6) It's still not used anywhere in the tree (tho local_t is), so there's no
    reason to stick to the current semantics.

    > 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.

    Yep, but this one is different from the SNMP stats which needs softirq vs
    user context safety. This is where I start wondering how many interfaces
    we're going to have...

    Sorry to add more questions than answers :(
    Rusty.
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