Subject: Re: [PATCH v2 3/3] virtio-net: enable virtio indirect cache

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 1 Nov 2021 04:33:38 -0400

On Thu, Oct 28, 2021 at 06:49:19PM +0800, Xuan Zhuo wrote:
> If the VIRTIO_RING_F_INDIRECT_DESC negotiation succeeds, and the number
> of sgs used for sending packets is greater than 1. We must constantly
> call __kmalloc/kfree to allocate/release desc.
> 
> In the case of extremely fast package delivery, the overhead cannot be
> ignored:
> 
>   27.46%  [kernel]  [k] virtqueue_add
>   16.66%  [kernel]  [k] detach_buf_split
>   16.51%  [kernel]  [k] virtnet_xsk_xmit
>   14.04%  [kernel]  [k] virtqueue_add_outbuf
>    5.18%  [kernel]  [k] __kmalloc
>    4.08%  [kernel]  [k] kfree
>    2.80%  [kernel]  [k] virtqueue_get_buf_ctx
>    2.22%  [kernel]  [k] xsk_tx_peek_desc
>    2.08%  [kernel]  [k] memset_erms
>    0.83%  [kernel]  [k] virtqueue_kick_prepare
>    0.76%  [kernel]  [k] virtnet_xsk_run
>    0.62%  [kernel]  [k] __free_old_xmit_ptr
>    0.60%  [kernel]  [k] vring_map_one_sg
>    0.53%  [kernel]  [k] native_apic_mem_write
>    0.46%  [kernel]  [k] sg_next
>    0.43%  [kernel]  [k] sg_init_table
>    0.41%  [kernel]  [k] kmalloc_slab
> 
> Compared to not using virtio indirect cache, virtio-net can get a 16%
> performance improvement when using indirect desc cache.
> 
> In the test case, the CPU where the package is sent has reached 100%.
> The following are the PPS in two cases:
> 
>     indirect desc cache  | no cache
>     3074658              | 2685132
>     3111866              | 2666118
>     3152527              | 2653632
>     3125867              | 2669820
>     3027147              | 2644464
>     3069211              | 2669777
>     3038522              | 2675645
>     3034507              | 2671302
>     3102257              | 2685504
>     3083712              | 2692800
>     3051771              | 2676928
>     3080684              | 2695040
>     3147816              | 2720876
>     3123887              | 2705492
>     3180963              | 2699520
>     3191579              | 2676480
>     3161670              | 2686272
>     3189768              | 2692588
>     3174272              | 2686692
>     3143434              | 2682416
> 
> Signed-off-by: Xuan Zhuo <[email-masked]>
> ---
>  drivers/net/virtio_net.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 4ad25a8b0870..e1ade176ab46 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -31,6 +31,13 @@ module_param(csum, bool, 0444);
>  module_param(gso, bool, 0444);
>  module_param(napi_tx, bool, 0644);
>  
> +/**
> + * Because virtio desc cache will increase memory overhead, users can turn it
> + * off or select an acceptable value. The maximum value is 2 + MAX_SKB_FRAGS.
> + */

Maybe add code to validate it and cap it at acceptable values then.

> +static u32 virtio_desc_cache_thr = 4;

Wouldn't something like CACHE_LINE_SIZE make more sense here?

> +module_param(virtio_desc_cache_thr, uint, 0644);
> +
>  /* FIXME: MTU in config. */
>  #define GOOD_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
>  #define GOOD_COPY_LEN	128

> @@ -3214,6 +3221,11 @@ static int virtnet_probe(struct virtio_device *vdev)
>  		vi->curr_queue_pairs = num_online_cpus();
>  	vi->max_queue_pairs = max_queue_pairs;
>  
> +	if (virtio_desc_cache_thr > 2 + MAX_SKB_FRAGS)
> +		virtio_set_desc_cache(vdev, 2 + MAX_SKB_FRAGS);
> +	else
> +		virtio_set_desc_cache(vdev, virtio_desc_cache_thr);
> +
>  	/* Allocate/initialize the rx/tx queues, and invoke find_vqs */
>  	err = init_vqs(vi);
>  	if (err)
> -- 
> 2.31.0