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

[Calvin Owens <calvinowens@fb.com>]

On Wednesday 01/07 at 08:52 -0800, Paul E. McKenney wrote:
> 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?

That's great, thanks :)

Should this go to stable as well? It is technically a regression, albeit
a rather long-standing one.

My original scenario was a bit contrived, but I tested this on some real
loads and it makes a difference: on a heavily loaded Proxygen server,
the aggregate softirq CPU usage decreases by roughly 10% (relative)
given the same amount of traffic with the patch. It also produces
statistically significant performance wins at higher loads on
webservers: about a 1% reduction in overall CPU utilization and improved
latency metrics.

Thanks,
Calvin
 
> 							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();
>