Re: [PATCH net-next v4 10/10] virtio-net: xsk zero copy xmit kick by threshold

[Jason Wang <jasowang@redhat.com>]

在 2021/4/13 上午11:15, Xuan Zhuo 写道:
> After testing, the performance of calling kick every time is not stable.
> And if all the packets are sent and kicked again, the performance is not
> good. So add a module parameter to specify how many packets are sent to
> call a kick.
>
> 8 is a relatively stable value with the best performance.
>
> Here is the pps of the test of xsk_kick_thr under different values (from
> 1 to 64).
>
> thr  PPS             thr PPS             thr PPS
> 1    2924116.74247 | 23  3683263.04348 | 45  2777907.22963
> 2    3441010.57191 | 24  3078880.13043 | 46  2781376.21739
> 3    3636728.72378 | 25  2859219.57656 | 47  2777271.91304
> 4    3637518.61468 | 26  2851557.9593  | 48  2800320.56575
> 5    3651738.16251 | 27  2834783.54408 | 49  2813039.87599
> 6    3652176.69231 | 28  2847012.41472 | 50  3445143.01839
> 7    3665415.80602 | 29  2860633.91304 | 51  3666918.01281
> 8    3665045.16555 | 30  2857903.5786  | 52  3059929.2709
> 9    3671023.2401  | 31  2835589.98963 | 53  2831515.21739
> 10   3669532.23274 | 32  2862827.88706 | 54  3451804.07204
> 11   3666160.37749 | 33  2871855.96696 | 55  3654975.92385
> 12   3674951.44813 | 34  3434456.44816 | 56  3676198.3188
> 13   3667447.57331 | 35  3656918.54177 | 57  3684740.85619
> 14   3018846.0503  | 36  3596921.16722 | 58  3060958.8594
> 15   2792773.84505 | 37  3603460.63903 | 59  2828874.57191
> 16   3430596.3602  | 38  3595410.87666 | 60  3459926.11027
> 17   3660525.85806 | 39  3604250.17819 | 61  3685444.47599
> 18   3045627.69054 | 40  3596542.28428 | 62  3049959.0809
> 19   2841542.94177 | 41  3600705.16054 | 63  2806280.04013
> 20   2830475.97348 | 42  3019833.71191 | 64  3448494.3913
> 21   2845655.55789 | 43  2752951.93264 |
> 22   3450389.84365 | 44  2753107.27164 |
>
> It can be found that when the value of xsk_kick_thr is relatively small,
> the performance is not good, and when its value is greater than 13, the
> performance will be more irregular and unstable. It looks similar from 3
> to 13, I chose 8 as the default value.
>
> The test environment is qemu + vhost-net. I modified vhost-net to drop
> the packets sent by vm directly, so that the cpu of vm can run higher.
> By default, the processes in the vm and the cpu of softirqd are too low,
> and there is no obvious difference in the test data.
>
> During the test, the cpu of softirq reached 100%. Each xsk_kick_thr was
> run for 300s, the pps of every second was recorded, and the average of
> the pps was finally taken. The vhost process cpu on the host has also
> reached 100%.
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> Reviewed-by: Dust Li <dust.li@linux.alibaba.com>
> ---
>   drivers/net/virtio_net.c | 19 +++++++++++++++++--
>   1 file changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index c441d6bf1510..4e360bfc2cf0 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -28,9 +28,11 @@ static int napi_weight = NAPI_POLL_WEIGHT;
>   module_param(napi_weight, int, 0444);
>   
>   static bool csum = true, gso = true, napi_tx = true;
> +static int xsk_kick_thr = 8;
>   module_param(csum, bool, 0444);
>   module_param(gso, bool, 0444);
>   module_param(napi_tx, bool, 0644);
> +module_param(xsk_kick_thr, int, 0644);
>   
>   /* FIXME: MTU in config. */
>   #define GOOD_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
> @@ -2690,6 +2692,7 @@ static int virtnet_xsk_xmit_batch(struct send_queue *sq,
>   	struct xdp_desc desc;
>   	int err, packet = 0;
>   	int ret = -EAGAIN;
> +	int need_kick = 0;
>   
>   	if (sq->xsk.last_desc.addr) {
>   		if (sq->vq->num_free < 2 + MAX_SKB_FRAGS)
> @@ -2700,6 +2703,7 @@ static int virtnet_xsk_xmit_batch(struct send_queue *sq,
>   			return -EBUSY;
>   
>   		++packet;
> +		++need_kick;
>   		--budget;
>   		sq->xsk.last_desc.addr = 0;
>   	}
> @@ -2723,11 +2727,22 @@ static int virtnet_xsk_xmit_batch(struct send_queue *sq,
>   		}
>   
>   		++packet;
> +		++need_kick;
> +		if (need_kick > xsk_kick_thr) {
> +			if (virtqueue_kick_prepare(sq->vq) &&
> +			    virtqueue_notify(sq->vq))


I woner whether it's time to introduce a helper in the virtio core to do 
this.

Thanks


> +				++stats->kicks;
> +
> +			need_kick = 0;
> +		}
>   	}
>   
>   	if (packet) {
> -		if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq))
> -			++stats->kicks;
> +		if (need_kick) {
> +			if (virtqueue_kick_prepare(sq->vq) &&
> +			    virtqueue_notify(sq->vq))
> +				++stats->kicks;
> +		}
>   
>   		*done = packet;
>   		stats->xdp_tx += packet;