From e8e8a2e47db6bb85bb0cb21e77b5c6aaedf864b4 Mon Sep 17 00:00:00 2001 From: Andi Kleen Date: Wed, 28 Feb 2018 13:43:28 -0800 Subject: [PATCH] random: optimize add_interrupt_randomness 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 Signed-off-by: Theodore Ts'o --- drivers/char/random.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/drivers/char/random.c b/drivers/char/random.c index 11c23ca57430..ee0c0d18f1eb 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -709,7 +709,8 @@ static void credit_entropy_bits(struct entropy_store *r, int nbits) } /* 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); }