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    SubjectRe: [PATCH] percpu_counter: Put a reasonable upper bound on percpu_counter_batch

    Hi Ted,

    > Um, this was an ext4 patch and I pointed out it could cause
    > problems. (Specifically, data loss…)

    I'm a bit confused. While the comment mentions ext4, the patch is just
    putting an upper bound on the size of percpu_counter_batch and it is
    useful for percpu_counter_compare() too:

    static void compute_batch_value(void)
    {
    int nr = num_online_cpus();

    - percpu_counter_batch = max(32, nr*2);
    + /*
    + * The cutoff point for the percpu_counter_compare() fast path grows
    + * at num_online_cpus^2 and on a big enough machine it will be
    + * unlikely to hit.
    + * We clamp the batch value to 1024 so the cutoff point only grows
    + * linearly past 512 CPUs.
    + */
    + percpu_counter_batch = clamp(nr*2, 32, 1024);
    }

    The batch value should be opaque to the rest of the kernel. If ext4
    requires a specific batch value we can use the functions that take
    an explicit one (eg __percpu_counter_add).

    Anton
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