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

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>

diff --git a/kernel/softirq.c b/kernel/softirq.c
index d7d498d8cc4f8185954a220c66071cbc5761b917..b331c8756543a3c609829d0bc5cf2583dd5208fb 100644 (file)
--- 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();