Re: [PATCH 3.19 016/175] ksoftirqd: Enable IRQs and call cond_resched() before poking RCU

Josh Hunt
2015-05-01T20:00:04Z

On Wed, Mar 4, 2015 at 12:13 AM, Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
> 3.19-stable review patch. If anyone has any objections, please let me know.
>
> ------------------
>
> From: Calvin Owens <calvinowens@fb.com>
>
> 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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>
> ---
> kernel/softirq.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -656,9 +656,13 @@ static void run_ksoftirqd(unsigned int c
> * 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();
>
>
> --
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Sorry for the delay in noticing this, but should this be applied to
3.14-stable as well?

Thanks
Josh