ksoftirqd: Enable IRQs and call cond_resched() before poking RCU

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>
Cc: stable@vger.kernel.org
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 501baa9ac1be..c497fcdf0d1e 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -656,9 +656,13 @@ static void run_ksoftirqd(unsigned int cpu)
                  * 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();