On Tue, Jan 06, 2015 at 05:49:06PM -0800, 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 <[email protected]>
> ---
> 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();
If this is for 3.20, we can just replace cond_resched() with
cond_resched_rcu_qs(), and get rid of the direct call to
rcu_note_context_switch(). This has the benefit of avoiding
needless rcu_note_context_switch() overhead if cond_resched()
actually did a reschedule.
But don't try it in 3.19 or earlier. ;-)
Thanx, Paul
> +
> + preempt_disable();
> + rcu_note_context_switch();
> + preempt_enable();
> +
> return;
> }
> local_irq_enable();
> --
> 2.1.1
>
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