| From | Jiri Slaby <> | Subject | [PATCH 3.12 073/142] ksoftirqd: Enable IRQs and call cond_resched() before poking RCU | Date | Sat, 16 May 2015 09:37:13 +0200 |
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From: Calvin Owens <calvinowens@fb.com>
3.12-stable review patch. If anyone has any objections, please let me know.
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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> --- 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(); -- 2.3.7
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