From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753535AbbAGQwb (ORCPT ); Wed, 7 Jan 2015 11:52:31 -0500 Received: from e31.co.us.ibm.com ([32.97.110.149]:58852 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752525AbbAGQwa (ORCPT ); Wed, 7 Jan 2015 11:52:30 -0500 Date: Wed, 7 Jan 2015 08:52:24 -0800 From: "Paul E. McKenney" To: Calvin Owens Cc: Thomas Gleixner , Andrew Morton , Joe Perches , Peter Zijlstra , linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH v2] ksoftirqd: Enable IRQs and call cond_resched() before poking RCU Message-ID: <20150107165223.GA21555@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1420594659-16996-1-git-send-email-calvinowens@fb.com> <20150107014906.GA27996@mail.thefacebook.com> <20150107021926.GT5280@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150107021926.GT5280@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15010716-8236-0000-0000-000008699BB4 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 06, 2015 at 06:19:26PM -0800, Paul E. McKenney wrote: > On Tue, Jan 06, 2015 at 05:49:06PM -0800, Calvin Owens wrote: > > 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 > > --- > > Changes since v1: > > I mixed up the kernel versions I was patching against, sorry! > > > > 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(); > > If this is for 3.20, we can just replace cond_resched() with > cond_resched_rcu_qs(), and get rid of the direct call to > rcu_note_context_switch(). This has the benefit of avoiding > needless rcu_note_context_switch() overhead if cond_resched() > actually did a reschedule. > > But don't try it in 3.19 or earlier. ;-) As in the following for 3.20. Does this version work for you? Thanx, Paul ------------------------------------------------------------------------ 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, but the new cond_resched_rcu_qs() provides shorter code and avoids double RCU notification in the case where cond_resched() really did a context switch. Signed-off-by: Calvin Owens [ paulmck: Substituted shiny new cond_resched_rcu_qs() primitive. ] Signed-off-by: Paul E. McKenney diff --git a/kernel/softirq.c b/kernel/softirq.c index 501baa9ac1be..8cdb98847c7b 100644 --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -656,9 +656,8 @@ static void run_ksoftirqd(unsigned int cpu) * in the task stack here. */ __do_softirq(); - rcu_note_context_switch(); local_irq_enable(); - cond_resched(); + cond_resched_rcu_qs(); return; } local_irq_enable();