From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934776AbbCDHDm (ORCPT ); Wed, 4 Mar 2015 02:03:42 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:43344 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759468AbbCDGTr (ORCPT ); Wed, 4 Mar 2015 01:19:47 -0500 From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Calvin Owens , "Paul E. McKenney" Subject: [PATCH 3.19 016/175] ksoftirqd: Enable IRQs and call cond_resched() before poking RCU Date: Tue, 3 Mar 2015 22:13:14 -0800 Message-Id: <20150304061029.058110661@linuxfoundation.org> X-Mailer: git-send-email 2.3.1 In-Reply-To: <20150304061026.134125919@linuxfoundation.org> References: <20150304061026.134125919@linuxfoundation.org> User-Agent: quilt/0.64 MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 3.19-stable review patch. If anyone has any objections, please let me know. ------------------ From: Calvin Owens commit 28423ad283d5348793b0c45cc9b1af058e776fd6 upstream. While debugging an issue with excessive softirq usage, I encountered the following note in commit 3e339b5dae24a706 ("softirq: Use hotplug thread infrastructure"): [ paulmck: Call rcu_note_context_switch() with interrupts enabled. ] ...but despite this note, the patch still calls RCU with IRQs disabled. This seemingly innocuous change caused a significant regression in softirq CPU usage on the sending side of a large TCP transfer (~1 GB/s): when introducing 0.01% packet loss, the softirq usage would jump to around 25%, spiking as high as 50%. Before the change, the usage would never exceed 5%. Moving the call to rcu_note_context_switch() after the cond_sched() call, as it was originally before the hotplug patch, completely eliminated this problem. Signed-off-by: Calvin Owens Signed-off-by: Paul E. McKenney Signed-off-by: Greg Kroah-Hartman --- kernel/softirq.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -656,9 +656,13 @@ static void run_ksoftirqd(unsigned int c * in the task stack here. */ __do_softirq(); - rcu_note_context_switch(); local_irq_enable(); cond_resched(); + + preempt_disable(); + rcu_note_context_switch(); + preempt_enable(); + return; } local_irq_enable();