From 5f3167f9438660896456c907e4c8468db0407a6e Mon Sep 17 00:00:00 2001
From: Andi Kleen <ak@linux.intel.com>
Date: Wed, 28 Feb 2018 13:43:28 -0800
Subject: [PATCH] random: optimize add_interrupt_randomness
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(-)
diff --git a/drivers/char/random.c b/drivers/char/random.c
index 474f65128533..99a0f16d0541 100644
--- 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);
}