Re: [PATCH 3.19 016/175] ksoftirqd: Enable IRQs and call cond_resched() before poking RCU

Mike Galbraith
2015-05-02T03:14:53Z

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