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 @@ -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); }