From: Calvin Owens <calvinowens@xxxxxx>
This patch has been added to the 3.12 stable tree. If you have any
objections, please let us 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@xxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Mike Galbraith <mgalbraith@xxxxxxx>
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
---
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();
--
2.3.5
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