This is a note to let you know that I've just added the patch titled random: optimize add_interrupt_randomness to the 4.14-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: random-optimize-add_interrupt_randomness.patch and it can be found in the queue-4.14 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From foo@baz Thu Jun 16 07:08:33 PM CEST 2022 From: Andi Kleen <ak@xxxxxxxxxxxxxxx> Date: Wed, 28 Feb 2018 13:43:28 -0800 Subject: random: optimize add_interrupt_randomness From: Andi Kleen <ak@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx> Signed-off-by: Theodore Ts'o <tytso@xxxxxxx> Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- drivers/char/random.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -721,7 +721,8 @@ 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); } Patches currently in stable-queue which might be from ak@xxxxxxxxxxxxxxx are queue-4.14/random-optimize-add_interrupt_randomness.patch