Patch "ksoftirqd: Enable IRQs and call cond_resched() before poking RCU" has been added to the 3.10-stable tree

This is a note to let you know that I've just added the patch titled

ksoftirqd: Enable IRQs and call cond_resched() before poking RCU

to the 3.10-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
ksoftirqd-enable-irqs-and-call-cond_resched-before-poking-rcu.patch
and it can be found in the queue-3.10 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Calvin Owens <[email protected]>
Date: Tue, 13 Jan 2015 13:16:18 -0800
Subject: ksoftirqd: Enable IRQs and call cond_resched() before poking RCU

From: Calvin Owens <[email protected]>

commit 28423ad283d5348793b0c45cc9b1af058e776fd6 upstream.

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.

---
 kernel/softirq.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

@@ -774,9 +774,13 @@ static void run_ksoftirqd(unsigned int c
 	local_irq_disable();
 	if (local_softirq_pending()) {
 		__do_softirq();
-		rcu_note_context_switch(cpu);
 		local_irq_enable();
 		cond_resched();
+
+		preempt_disable();
+		rcu_note_context_switch(cpu);
+		preempt_enable();
+
 		return;
 	}
 	local_irq_enable();

Patches currently in stable-queue which might be from [email protected] are

queue-3.10/ksoftirqd-enable-irqs-and-call-cond_resched-before-poking-rcu.patch