Re: [virtio-net][PATCH] Don't arm tx hrtimer with a constant 500us each transmit

Rusty Russell
2007-12-18T00:01:12Z

On Wednesday 12 December 2007 23:54:00 Dor Laor wrote:
> commit 763769621d271d92204ed27552d75448587c1ac0
> Author: Dor Laor <dor.laor@qumranet.com>
> Date: Wed Dec 12 14:52:00 2007 +0200
>
> [virtio-net][PATCH] Don't arm tx hrtimer with a constant 50us each
> transmit
>
> The current start_xmit sets 500us hrtimer to kick the host.
> The problem is that if another xmit happens before the timer was
> fired then
> the first xmit will have to wait additional 500us.
> This patch does not re-arm the timer if there is existing one.
> This will shorten the latency for tx.

Hi Dor!

Yes, I pondered this when I wrote the code. On the one hand, it's a
low-probability pathological corner case, on the other, your patch reduces
the number of timer reprograms in the normal case.

So I've applied it, thanks!
Rusty.

>
> Signed-off-by: Dor Laor <dor.laor@qumranet.com>
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 7b051d5..8bb17d1
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -406,10 +405,10 @@ again:
> virtio_debug(vdebug, "%s: before calling kick %d\n",
> __FUNCTION__, __LINE__);
> vi->svq->vq_ops->kick(vi->svq);
> vi->out_num = 0;
> - } else {
> - vi->stats.hrtimer_starts++;
> - hrtimer_start(&vi->tx_timer, ktime_set(0,500000),
> - HRTIMER_MODE_REL);
> + } else if (!hrtimer_is_queued(&vi->tx_timer)) {
> + vi->stats.hrtimer_starts++;
> + hrtimer_start(&vi->tx_timer, ktime_set(0,500000),
> + HRTIMER_MODE_REL);
> }
> return 0;
> }