[PATCH 3.19 016/175] ksoftirqd: Enable IRQs and call cond_resched() before poking RCU

Greg Kroah-Hartman
2015-03-04T07:03:42Z

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: 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
@@ -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();