From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935167AbeB1Vnz (ORCPT ); Wed, 28 Feb 2018 16:43:55 -0500 Received: from mga06.intel.com ([134.134.136.31]:4582 "EHLO mga06.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S935147AbeB1Vnx (ORCPT ); Wed, 28 Feb 2018 16:43:53 -0500 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.47,406,1515484800"; d="scan'208";a="21849941" From: Andi Kleen To: tytso@mit.edu Cc: linux-kernel@vger.kernel.org, Andi Kleen Subject: [PATCH] random: Optimize add_interrupt_randomness Date: Wed, 28 Feb 2018 13:43:28 -0800 Message-Id: <20180228214328.23108-1-andi@firstfloor.org> X-Mailer: git-send-email 2.14.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Andi Kleen 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 --- 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 e5b3d3ba4660..64a897a2888f 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); } -- 2.14.3