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    SubjectRe: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync
    On 11/17, Jens Axboe wrote:
    > It works for me, but the overhead is still large. Before it would take
    > 8-12 jiffies for a synchronize_srcu() to complete without there actually
    > being any reader locks active, now it takes 2-3 jiffies. So it's
    > definitely faster, and as suspected the loss of two of three
    > synchronize_sched() cut down the overhead to a third.
    > It's still too heavy for me, by far the most calls I do to
    > synchronize_srcu() doesn't have any reader locks pending. I'm still a
    > big advocate of the fastpath srcu_readers_active() check. I can
    > understand the reluctance to make it the default, but for my case it's
    > "safe enough", so if we could either export srcu_readers_active() or
    > export a synchronize_srcu_fast() (or something like that), then SRCU
    > would be a good fit for barrier vs plug rework.

    Just an idea. How about another variant of srcu which is more optimized
    for writers?

    struct xxx_struct {
    int completed;
    atomic_t ctr[2];
    struct mutex mutex;
    wait_queue_head_t wq;

    void init_xxx_struct(struct xxx_struct *sp)
    sp->completed = 0;
    atomic_set(sp->ctr + 0, 1);
    atomic_set(sp->ctr + 1, 1);

    int xxx_read_lock(struct xxx_struct *sp)
    int idx;

    idx = sp->completed & 0x1;
    atomic_inc(sp->ctr + idx);

    return idx;

    void xxx_read_unlock(struct xxx_struct *sp, int idx)
    if (atomic_dec_and_test(sp->ctr + idx))

    void synchronize_xxx(struct xxx_struct *sp)
    wait_queue_t wait;
    int idx;


    idx = sp->completed++ & 0x1;

    for (;;) {
    prepare_to_wait(&sp->wq, &wait, TASK_UNINTERRUPTIBLE);

    if (!atomic_add_unless(sp->ctr + idx, -1, 1))

    atomic_inc(sp->ctr + idx);
    finish_wait(&sp->wq, &wait);


    Very simple. Note that synchronize_xxx() is O(1), doesn't poll, and could
    be optimized further.


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