Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751491AbbEBDOx (ORCPT ); Fri, 1 May 2015 23:14:53 -0400 Received: from mail-wi0-f177.google.com ([209.85.212.177]:35797 "EHLO mail-wi0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750823AbbEBDOu (ORCPT ); Fri, 1 May 2015 23:14:50 -0400 Message-ID: <1430536490.3137.30.camel@gmail.com> Subject: Re: [PATCH 3.19 016/175] ksoftirqd: Enable IRQs and call cond_resched() before poking RCU From: Mike Galbraith To: Greg Kroah-Hartman Cc: Josh Hunt , LKML , stable , Calvin Owens , "Paul E. McKenney" Date: Sat, 02 May 2015 05:14:50 +0200 In-Reply-To: <20150501205205.GC2265@kroah.com> References: <20150304061026.134125919@linuxfoundation.org> <20150304061029.058110661@linuxfoundation.org> <20150501205205.GC2265@kroah.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.12.11 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4729 Lines: 128 On Fri, 2015-05-01 at 22:52 +0200, Greg Kroah-Hartman wrote: > On Fri, May 01, 2015 at 03:00:00PM -0500, Josh Hunt wrote: > > On Wed, Mar 4, 2015 at 12:13 AM, Greg Kroah-Hartman > > wrote: > > > 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(); > > > > > > > > > -- > > > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > > > the body of a message to majordomo@vger.kernel.org > > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > > Please read the FAQ at http://www.tux.org/lkml/ > > > > Sorry for the delay in noticing this, but should this be applied to > > 3.14-stable as well? > > Why should it? The regression inducing change arrived in 3.7-rc1. > And odds are, if I didn't apply it there, it was either > because it didn't apply, or it broke the build. a. [x] rcu_note_context_switch(cpu) -> rcu_note_context_switch() >From 28423ad283d5348793b0c45cc9b1af058e776fd6 Mon Sep 17 00:00:00 2001 From: Calvin Owens Date: Tue, 13 Jan 2015 13:16:18 -0800 Subject: ksoftirqd: Enable IRQs and call cond_resched() before poking RCU 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 Cc: stable@vger.kernel.org Signed-off-by: Paul E. McKenney Signed-off-by: Mike Galbraith --- kernel/softirq.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -657,9 +657,13 @@ static void run_ksoftirqd(unsigned int c * in the task stack here. */ __do_softirq(); - rcu_note_context_switch(cpu); local_irq_enable(); cond_resched(); + + preempt_disable(); + rcu_note_context_switch(cpu); + preempt_enable(); + return; } local_irq_enable(); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/