From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756584AbbAGBhu (ORCPT ); Tue, 6 Jan 2015 20:37:50 -0500 Received: from mx0b-00082601.pphosted.com ([67.231.153.30]:13743 "EHLO mx0a-00082601.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751845AbbAGBhs (ORCPT ); Tue, 6 Jan 2015 20:37:48 -0500 From: Calvin Owens To: Thomas Gleixner , Andrew Morton , Joe Perches , "Paul E. McKenney" , Peter Zijlstra CC: , , Calvin Owens Subject: [PATCH] ksoftirqd: Enable IRQs and call cond_resched() before poking RCU Date: Tue, 6 Jan 2015 17:37:39 -0800 Message-ID: <1420594659-16996-1-git-send-email-calvinowens@fb.com> X-Mailer: git-send-email 2.1.1 MIME-Version: 1.0 Content-Type: text/plain X-Originating-IP: [192.168.16.4] X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:5.13.68,1.0.33,0.0.0000 definitions=2015-01-07_01:2015-01-06,2015-01-07,1970-01-01 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 kscore.is_bulkscore=0 kscore.compositescore=0 circleOfTrustscore=3.45033029023803 compositescore=0.928708196901072 urlsuspect_oldscore=0.928708196901072 suspectscore=0 recipient_domain_to_sender_totalscore=0 phishscore=0 bulkscore=0 kscore.is_spamscore=0 recipient_to_sender_totalscore=0 recipient_domain_to_sender_domain_totalscore=64355 rbsscore=0.928708196901072 spamscore=0 recipient_to_sender_domain_totalscore=12 urlsuspectscore=0.9 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=7.0.1-1402240000 definitions=main-1501070016 X-FB-Internal: deliver Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- kernel/softirq.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/kernel/softirq.c b/kernel/softirq.c index 501baa9..9e787d8 100644 --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -656,9 +656,13 @@ static void run_ksoftirqd(unsigned int cpu) * in the task stack here. */ __do_softirq(); - rcu_note_context_switch(); local_irq_enable(); cond_resched(); + + preempt_disable(); + rcu_note_context_switch(cpu); + preempt_enable(); + return; } local_irq_enable(); -- 2.1.1