Re: [PATCH] rps: send IPIs ASAP

[Tom Herbert <therbert@google.com>]

On Mon, Apr 19, 2010 at 9:08 PM, Changli Gao <xiaosuo@gmail.com> wrote:
> rps: send IPIs ASAP
>
> In order to reduce latency, we'd better send IPIs ASAP to schedule the
> corresponding NAPIs.
>
A design point of RPS is that we generate at most one IPI per CPU per
device interrupt, which at least offers some predictable coalescing.
With your changes, we would get at most one IPI per packet-- that
could represent a lot more of them.  Did you test this to see what the
impact is in this regard?


> For NAPI drivers, we send IPIs immediately, and for the others, we defer them
> to NET_RX_SOFTIRQ. In this patch, we move net_rps_action() to the beginning of
> net_rx_action() to emulate a softirq with the higher priority than
> NET_RX_SOFTIRQ.
>
> Signed-off-by: Changli Gao <xiaosuo@gmail.com>
> ----
>  net/core/dev.c |   23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 05a2b29..d8fca21 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -2363,6 +2363,10 @@ static int enqueue_to_backlog(struct sk_buff *skb, int cpu,
>  {
>        struct softnet_data *queue;
>        unsigned long flags;
> +#ifdef CONFIG_RPS
> +       bool ni = !in_irq();
> +       bool ipi = false;
> +#endif
>
>        queue = &per_cpu(softnet_data, cpu);
>
> @@ -2380,6 +2384,10 @@ enqueue:
>  #endif
>                        rps_unlock(queue);
>                        local_irq_restore(flags);
> +#ifdef CONFIG_RPS
> +                       if (ipi)
> +                               __smp_call_function_single(cpu, &queue->csd, 0);
> +#endif
>                        return NET_RX_SUCCESS;
>                }
>
> @@ -2389,6 +2397,11 @@ enqueue:
>                        if (cpu != smp_processor_id()) {
>                                struct softnet_data *myqueue;
>
> +                               if (ni) {
> +                                       ipi = true;
> +                                       goto enqueue;
> +                               }
> +
>                                myqueue = &__get_cpu_var(softnet_data);
>                                queue->rps_ipi_next = myqueue->rps_ipi_list;
>                                myqueue->rps_ipi_list = queue;
> @@ -3337,6 +3350,7 @@ static void net_rps_action(void)
>        struct softnet_data *locqueue = &__get_cpu_var(softnet_data);
>        struct softnet_data *remqueue = locqueue->rps_ipi_list;
>
> +       local_irq_disable();
>        if (remqueue) {
>                locqueue->rps_ipi_list = NULL;
>
> @@ -3350,9 +3364,10 @@ static void net_rps_action(void)
>                                                           &remqueue->csd, 0);
>                        remqueue = next;
>                }
> -       } else
> -#endif
> +       } else {
>                local_irq_enable();
> +       }
> +#endif
>  }
>
>  static void net_rx_action(struct softirq_action *h)
> @@ -3362,6 +3377,8 @@ static void net_rx_action(struct softirq_action *h)
>        int budget = netdev_budget;
>        void *have;
>
> +       net_rps_action();
> +
>        local_irq_disable();
>
>        while (!list_empty(list)) {
> @@ -3423,7 +3440,7 @@ static void net_rx_action(struct softirq_action *h)
>                netpoll_poll_unlock(have);
>        }
>  out:
> -       net_rps_action();
> +       local_irq_enable();
>
>  #ifdef CONFIG_NET_DMA
>        /*
>