Re: [PATCH v2] ksoftirqd: Enable IRQs and call cond_resched() before poking RCU

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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 <calvinowens@fb.com>
> > ---
> > 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 <calvinowens@fb.com>
[ paulmck: Substituted shiny new cond_resched_rcu_qs() primitive. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

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();