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Date: Thu, 8 Jan 2015 13:46:44 -0800
From: Calvin Owens <calvinowens@...com>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
CC: Thomas Gleixner <tglx@...utronix.de>,
Andrew Morton <akpm@...ux-foundation.org>,
Joe Perches <joe@...ches.com>,
Peter Zijlstra <peterz@...radead.org>,
<linux-kernel@...r.kernel.org>, <kernel-team@...com>
Subject: Re: [PATCH v2] ksoftirqd: Enable IRQs and call cond_resched() before
poking RCU
On Wednesday 01/07 at 20:53 -0800, Paul E. McKenney wrote:
> On Wed, Jan 07, 2015 at 08:33:29PM -0800, Calvin Owens wrote:
> > 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@...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.
>
> Your original patch could go into stable, but my updated version depends
> on functionality not present before 3.20.
Right. I'll wait until 3.20-rc1, and then I'll send the original to
stable with a reference to the commit.
> > 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.
>
> OK, good to know. I have added this information to the commit log, please
> see below for updated commit.
Looks good!
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%. 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.
>
> 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@...com>
> [ paulmck: Substituted shiny new cond_resched_rcu_qs() primitive. ]
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> [ paulmck: Added Calvin's measurements on Proxygen server and webservers. ]
>
> 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();
>
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