netdev - [PATCH v4 3/3] virtio-net: enable virtio desc cache

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Date:   Mon,  8 Nov 2021 19:49:51 +0800
From:   Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Subject: [PATCH v4 3/3] virtio-net: enable virtio desc cache

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 virtio 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 <xuanzhuo@...ux.alibaba.com>
---
 drivers/net/virtio_net.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 9ff2ef9dceca..193c8b38433e 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -42,6 +42,9 @@ module_param(csum, bool, 0444);
 module_param(gso, bool, 0444);
 module_param(napi_tx, bool, 0644);
 
+static u32 virtio_desc_cache_threshold = MAX_SKB_FRAGS + 2;
+module_param(virtio_desc_cache_threshold, uint, 0644);
+
 /* FIXME: MTU in config. */
 #define GOOD_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
 #define GOOD_COPY_LEN	128
@@ -3350,10 +3353,10 @@ static unsigned int mergeable_min_buf_len(struct virtnet_info *vi, struct virtqu
 
 static int virtnet_find_vqs(struct virtnet_info *vi)
 {
+	int i, total_vqs, threshold;
 	vq_callback_t **callbacks;
 	struct virtqueue **vqs;
 	int ret = -ENOMEM;
-	int i, total_vqs;
 	const char **names;
 	bool *ctx;
 
@@ -3411,10 +3414,17 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
 			vi->dev->features |= NETIF_F_HW_VLAN_CTAG_FILTER;
 	}
 
+	threshold = min_t(u32, virtio_desc_cache_threshold, 2 + MAX_SKB_FRAGS);
+
 	for (i = 0; i < vi->max_queue_pairs; i++) {
 		vi->rq[i].vq = vqs[rxq2vq(i)];
 		vi->rq[i].min_buf_len = mergeable_min_buf_len(vi, vi->rq[i].vq);
 		vi->sq[i].vq = vqs[txq2vq(i)];
+
+		if (!vi->mergeable_rx_bufs && vi->big_packets)
+			virtqueue_set_desc_cache(vi->rq[i].vq, MAX_SKB_FRAGS + 2);
+
+		virtqueue_set_desc_cache(vi->sq[i].vq, threshold);
 	}
 
 	/* run here: ret == 0. */
-- 
2.31.0