linux-kernel - [PATCH 3.19 016/175] ksoftirqd: Enable IRQs and call cond

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Message-Id: <20150304061029.058110661@linuxfoundation.org>
Date:	Tue,  3 Mar 2015 22:13:14 -0800
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Calvin Owens <calvinowens@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH 3.19 016/175] ksoftirqd: Enable IRQs and call cond_resched() before poking RCU

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

------------------

From: Calvin Owens <calvinowens@...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@...com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

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

--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -656,9 +656,13 @@ static void run_ksoftirqd(unsigned int c
 		 * in the task stack here.
 		 */
 		__do_softirq();
-		rcu_note_context_switch();
 		local_irq_enable();
 		cond_resched();
+
+		preempt_disable();
+		rcu_note_context_switch();
+		preempt_enable();
+
 		return;
 	}
 	local_irq_enable();

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