Re: [PATCH] ksoftirqd: Enable IRQs and call cond_resched() before poking RCU

[Thomas Gleixner <tglx@linutronix.de>]

On Tue, 13 Jan 2015, Calvin Owens wrote:

> 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
> ---
> This version includes the "cpu" argument to rcu_note_context_switch() in
> order to apply cleanly to stable kernels. It will need to be removed to
> apply to 3.18+ and 3.19 (upstream commit 38200cf2 removed the argument).
> 
>  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(cpu);
>  		local_irq_enable();
>  		cond_resched();
> +
> +		preempt_disable();
> +		rcu_note_context_switch(cpu);
> +		preempt_enable();
> +

The whole rcu_note_context_switch() in run_ksoftirqd() is silly.

    cond_resched()
	__preempt_count_add(PREEMPT_ACTIVE);

	__schedule();
	     preempt_disable();
	     rcu_note_context_switch();
	     ....

	__preempt_count_sub(PREEMPT_ACTIVE);

Thanks,

	tglx