Messages in this thread Patch in this message | | | Date | Tue, 6 Jan 2015 17:49:06 -0800 | From | Calvin Owens <> | Subject | [PATCH v2] ksoftirqd: Enable IRQs and call cond_resched() before poking RCU |
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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> --- Changes since v1: I mixed up the kernel versions I was patching against, sorry!
kernel/softirq.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/kernel/softirq.c b/kernel/softirq.c index 501baa9..9e787d8 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(); -- 2.1.1
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