[PATCH 3.12 073/142] ksoftirqd: Enable IRQs and call cond_resched() before poking RCU

Jiri Slaby
2015-05-16T08:05:51Z

From: Calvin Owens <calvinowens@fb.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@fb.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Mike Galbraith <mgalbraith@suse.de>
Signed-off-by: Jiri Slaby <jslaby@suse.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