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    Subject[PATCH 4.9 028/264] random: optimize add_interrupt_randomness
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    From: Andi Kleen <ak@linux.intel.com>

    commit e8e8a2e47db6bb85bb0cb21e77b5c6aaedf864b4 upstream.

    add_interrupt_randomess always wakes up
    code blocking on /dev/random. This wake up is done
    unconditionally. Unfortunately this means all interrupts
    take the wait queue spinlock, which can be rather expensive
    on large systems processing lots of interrupts.

    We saw 1% cpu time spinning on this on a large macro workload
    running on a large system.

    I believe it's a recent regression (?)

    Always check if there is a waiter on the wait queue
    before waking up. This check can be done without
    taking a spinlock.

    1.06% 10460 [kernel.vmlinux] [k] native_queued_spin_lock_slowpath
    |
    ---native_queued_spin_lock_slowpath
    |
    --0.57%--_raw_spin_lock_irqsave
    |
    --0.56%--__wake_up_common_lock
    credit_entropy_bits
    add_interrupt_randomness
    handle_irq_event_percpu
    handle_irq_event
    handle_edge_irq
    handle_irq
    do_IRQ
    common_interrupt

    Signed-off-by: Andi Kleen <ak@linux.intel.com>
    Signed-off-by: Theodore Ts'o <tytso@mit.edu>
    Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/char/random.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    --- a/drivers/char/random.c
    +++ b/drivers/char/random.c
    @@ -722,7 +722,8 @@ retry:
    }

    /* should we wake readers? */
    - if (entropy_bits >= random_read_wakeup_bits) {
    + if (entropy_bits >= random_read_wakeup_bits &&
    + wq_has_sleeper(&random_read_wait)) {
    wake_up_interruptible(&random_read_wait);
    kill_fasync(&fasync, SIGIO, POLL_IN);
    }

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