linux-kernel - Re: [PATCH 3.19 016/175] ksoftirqd: Enable IRQs and call cond

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Date:	Sat, 02 May 2015 05:14:50 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Josh Hunt <joshhunt00@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	stable <stable@...r.kernel.org>,
	Calvin Owens <calvinowens@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 3.19 016/175] ksoftirqd: Enable IRQs and call
 cond_resched() before poking RCU

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
> > <gregkh@...uxfoundation.org> wrote:
> > > 3.19-stable review patch.  If anyone has any objections, please let me know.
> > >
> > > ------------------
> > >
> > > From: Calvin Owens <calvinowens@...com>
> > >
> > > 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 <calvinowens@...com>
> > > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > >
> > > ---
> > >  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@...r.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 <calvinowens@...com>
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 <calvinowens@...com>
Cc: stable@...r.kernel.org
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Signed-off-by: Mike Galbraith <mgalbraith@...e.de>
---
 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();


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