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

[<gregkh@linuxfoundation.org>]

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 <stable@vger.kernel.org> know about it.


>From 28423ad283d5348793b0c45cc9b1af058e776fd6 Mon Sep 17 00:00:00 2001
From: Calvin Owens <calvinowens@fb.com>
Date: Tue, 13 Jan 2015 13:16:18 -0800
Subject: ksoftirqd: Enable IRQs and call cond_resched() before poking RCU

From: Calvin Owens <calvinowens@fb.com>

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.

Signed-off-by: Calvin Owens <calvinowens@fb.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Mike Galbraith <mgalbraith@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 kernel/softirq.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -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 calvinowens@fb.com are

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