From: Calvin Owens Date: Tue, 13 Jan 2015 21:16:18 +0000 (-0800) Subject: ksoftirqd: Enable IRQs and call cond_resched() before poking RCU X-Git-Tag: firefly_0821_release~176^2~2265^2^2~1^2~1 X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=28423ad283d5348793b0c45cc9b1af058e776fd6;p=firefly-linux-kernel-4.4.55.git 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 Cc: stable@vger.kernel.org Signed-off-by: Paul E. McKenney --- 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();