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 <[email protected]>
> > ---
> > 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 <[email protected]>
[ paulmck: Substituted shiny new cond_resched_rcu_qs() primitive. ]
Signed-off-by: Paul E. McKenney <[email protected]>
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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