Messages in this thread | | | Date | Tue, 6 Jan 2015 18:19:26 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v2] ksoftirqd: Enable IRQs and call cond_resched() before poking RCU |
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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. ;-)
Thanx, Paul
> + > + preempt_disable(); > + rcu_note_context_switch(); > + preempt_enable(); > + > return; > } > local_irq_enable(); > -- > 2.1.1 >
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