linux-kernel - [PATCH 3.12 073/142] ksoftirqd: Enable IRQs and call cond

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Message-Id: <4e75ff4f0985418801dd55d90208dcf989f3c9fd.1431761807.git.jslaby@suse.cz>
Date:	Sat, 16 May 2015 09:37:13 +0200
From:	Jiri Slaby <jslaby@...e.cz>
To:	stable@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, Calvin Owens <calvinowens@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <mgalbraith@...e.de>,
	Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 073/142] ksoftirqd: Enable IRQs and call cond_resched() before poking RCU

From: Calvin Owens <calvinowens@...com>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

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@...com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Signed-off-by: Mike Galbraith <mgalbraith@...e.de>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 kernel/softirq.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index d7d498d8cc4f..b331c8756543 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -772,9 +772,13 @@ static void run_ksoftirqd(unsigned int cpu)
 	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();
-- 
2.3.7

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