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

[Mike Galbraith]

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@xxxxxxxxxxxxxxxxxxx> wrote:
> > > 3.19-stable review patch.  If anyone has any objections, please let me know.
> > >
> > > ------------------
> > >
> > > From: Calvin Owens <calvinowens@xxxxxx>
> > >
> > > 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@xxxxxx>
> > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > >
> > > ---
> > >  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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> > 
> > 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()