Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752009AbbEAUAE (ORCPT ); Fri, 1 May 2015 16:00:04 -0400 Received: from mail-oi0-f41.google.com ([209.85.218.41]:32929 "EHLO mail-oi0-f41.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751087AbbEAUAB (ORCPT ); Fri, 1 May 2015 16:00:01 -0400 MIME-Version: 1.0 In-Reply-To: <20150304061029.058110661@linuxfoundation.org> References: <20150304061026.134125919@linuxfoundation.org> <20150304061029.058110661@linuxfoundation.org> Date: Fri, 1 May 2015 15:00:00 -0500 Message-ID: Subject: Re: [PATCH 3.19 016/175] ksoftirqd: Enable IRQs and call cond_resched() before poking RCU From: Josh Hunt To: Greg Kroah-Hartman Cc: LKML , stable , Calvin Owens , "Paul E. McKenney" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2499 Lines: 70 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? Thanks Josh -- 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/