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    SubjectRe: [patch] reduce IPI noise due to /dev/cdrom open/close
    Date

    On Jul 4, 2006, at 1:39 AM, Andrew Morton wrote:

    >
    > I expect raw_smp_processor_id() is used here as a a microoptimisation -
    > avoid a might_sleep() which obviously will never trigger.
    >
    > But I think it'd be better to do just a single raw_smp_processor_id()
    > for this entire function:
    >
    > static void bh_lru_install(struct buffer_head *bh)
    > {
    > struct buffer_head *evictee = NULL;
    > struct bh_lru *lru;
    > + int cpu;
    >
    > check_irqs_on();
    > bh_lru_lock();
    > + cpu = raw_smp_processor_id();
    > - lru = &__get_cpu_var(bh_lrus);
    > + lru = per_cpu(bh_lrus, cpu);
    >

    The problem with this style is that it is an disoptimizatoin for
    architectures who hold their per-cpu data offset in a register, put the
    smp_processor_id in the per-cpu data (or similar) and per_cpu data
    offsets in a global lookup.

    Do we need a new macro? (what is that gcc macro function syntax?)

    #ifdef PER_CPU_IS_SLOW
    #define my_cpu_var(bh_lrus, cpu) \
    ({ BUG_ON(cpu != smp_processor_id()); &__get_cpu_var(bh_lrus); })
    #else
    #define my_cpu_var(bh_lrus, cpu) \
    ({ BUG_ON(cpu != smp_processor_id()); per_cpu(bh_lrus, cpu); })
    #endif

    (and yes, the BUG_ON would be for debug checking).

    milton

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