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    Subject[PATCH v2 1/2] virtio-ring: Use threshold for switching to indirect descriptors
    Date
    Currently if VIRTIO_RING_F_INDIRECT_DESC is enabled we will use indirect
    descriptors even if we have plenty of space in the ring. This means that
    we take a performance hit at all times due to the overhead of creating
    indirect descriptors.

    Instead, use it only after we're below a configurable offset.

    Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
    ---
    drivers/block/virtio_blk.c | 4 ++++
    drivers/char/hw_random/virtio-rng.c | 4 ++++
    drivers/char/virtio_console.c | 4 ++++
    drivers/net/virtio_net.c | 4 ++++
    drivers/virtio/virtio_balloon.c | 4 ++++
    drivers/virtio/virtio_ring.c | 21 +++++++++++++++------
    include/linux/virtio.h | 1 +
    net/9p/trans_virtio.c | 4 ++++
    8 files changed, 40 insertions(+), 6 deletions(-)

    diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
    index 2edfb5c..13b8ae9 100644
    --- a/drivers/block/virtio_blk.c
    +++ b/drivers/block/virtio_blk.c
    @@ -22,6 +22,9 @@ static DEFINE_IDA(vd_index_ida);

    struct workqueue_struct *virtblk_wq;

    +static unsigned int indirect_thresh;
    +module_param(indirect_thresh, uint, S_IRUGO);
    +
    struct virtio_blk
    {
    struct virtio_device *vdev;
    @@ -735,6 +738,7 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)

    INIT_WORK(&vblk->config_work, virtblk_config_changed_work);
    vblk->config_enable = true;
    + vdev->indirect_thresh = indirect_thresh;

    err = init_vq(vblk);
    if (err)
    diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
    index 5708299..02d8421 100644
    --- a/drivers/char/hw_random/virtio-rng.c
    +++ b/drivers/char/hw_random/virtio-rng.c
    @@ -25,6 +25,9 @@
    #include <linux/virtio_rng.h>
    #include <linux/module.h>

    +static unsigned int indirect_thresh;
    +module_param(indirect_thresh, uint, S_IRUGO);
    +
    static struct virtqueue *vq;
    static unsigned int data_avail;
    static DECLARE_COMPLETION(have_data);
    @@ -93,6 +96,7 @@ static int probe_common(struct virtio_device *vdev)
    int err;

    /* We expect a single virtqueue. */
    + vdev->indirect_thresh = indirect_thresh;
    vq = virtio_find_single_vq(vdev, random_recv_done, "input");
    if (IS_ERR(vq))
    return PTR_ERR(vq);
    diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
    index e88f843..fc14e7f 100644
    --- a/drivers/char/virtio_console.c
    +++ b/drivers/char/virtio_console.c
    @@ -39,6 +39,9 @@
    #include <linux/module.h>
    #include "../tty/hvc/hvc_console.h"

    +static unsigned int indirect_thresh;
    +module_param(indirect_thresh, uint, S_IRUGO);
    +
    /*
    * This is a global struct for storing common data for all the devices
    * this driver handles.
    @@ -1887,6 +1890,7 @@ static int __devinit virtcons_probe(struct virtio_device *vdev)
    max_nr_ports),
    &portdev->config.max_nr_ports) == 0)
    multiport = true;
    + vdev->indirect_thresh = indirect_thresh;

    err = init_vqs(portdev);
    if (err < 0) {
    diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
    index cbf8b06..64a8321 100644
    --- a/drivers/net/virtio_net.c
    +++ b/drivers/net/virtio_net.c
    @@ -34,6 +34,9 @@ static bool csum = true, gso = true;
    module_param(csum, bool, 0444);
    module_param(gso, bool, 0444);

    +static unsigned int indirect_thresh;
    +module_param(indirect_thresh, uint, S_IRUGO);
    +
    /* FIXME: MTU in config. */
    #define MAX_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
    #define GOOD_COPY_LEN 128
    @@ -1128,6 +1131,7 @@ static int virtnet_probe(struct virtio_device *vdev)

    if (virtio_has_feature(vdev, VIRTIO_NET_F_MRG_RXBUF))
    vi->mergeable_rx_bufs = true;
    + vdev->indirect_thresh = indirect_thresh;

    err = init_vqs(vi);
    if (err)
    diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
    index 0908e60..fce7347 100644
    --- a/drivers/virtio/virtio_balloon.c
    +++ b/drivers/virtio/virtio_balloon.c
    @@ -35,6 +35,9 @@
    */
    #define VIRTIO_BALLOON_PAGES_PER_PAGE (PAGE_SIZE >> VIRTIO_BALLOON_PFN_SHIFT)

    +static unsigned int indirect_thresh;
    +module_param(indirect_thresh, uint, S_IRUGO);
    +
    struct virtio_balloon
    {
    struct virtio_device *vdev;
    @@ -356,6 +359,7 @@ static int virtballoon_probe(struct virtio_device *vdev)
    init_waitqueue_head(&vb->acked);
    vb->vdev = vdev;
    vb->need_stats_update = 0;
    + vdev->indirect_thresh = indirect_thresh;

    err = init_vqs(vb);
    if (err)
    diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
    index 5aa43c3..99a64a7 100644
    --- a/drivers/virtio/virtio_ring.c
    +++ b/drivers/virtio/virtio_ring.c
    @@ -87,8 +87,11 @@ struct vring_virtqueue
    /* Other side has made a mess, don't try any more. */
    bool broken;

    - /* Host supports indirect buffers */
    - bool indirect;
    + /*
    + * Min. number of free space in the ring to trigger direct
    + * descriptor use
    + */
    + unsigned int indirect_thresh;

    /* Host publishes avail event idx */
    bool event;
    @@ -216,9 +219,12 @@ int virtqueue_add_buf(struct virtqueue *_vq,
    }
    #endif

    - /* If the host supports indirect descriptor tables, and we have multiple
    - * buffers, then go indirect. FIXME: tune this threshold */
    - if (vq->indirect && (out + in) > 1 && vq->num_free) {
    + /*
    + * If the host supports indirect descriptor tables, and we have multiple
    + * buffers, then go indirect.
    + */
    + if ((out + in) > 1 && vq->num_free &&
    + (vq->num_free < vq->indirect_thresh)) {
    head = vring_add_indirect(vq, sg, out, in, gfp);
    if (likely(head >= 0))
    goto add_head;
    @@ -647,13 +653,16 @@ struct virtqueue *vring_new_virtqueue(unsigned int num,
    vq->broken = false;
    vq->last_used_idx = 0;
    vq->num_added = 0;
    + vq->indirect_thresh = 0;
    list_add_tail(&vq->vq.list, &vdev->vqs);
    #ifdef DEBUG
    vq->in_use = false;
    vq->last_add_time_valid = false;
    #endif

    - vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC);
    + if (virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC))
    + vq->indirect_thresh = vdev->indirect_thresh;
    +
    vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);

    /* No callback? Tell other side not to bother us. */
    diff --git a/include/linux/virtio.h b/include/linux/virtio.h
    index a1ba8bb..48bc457 100644
    --- a/include/linux/virtio.h
    +++ b/include/linux/virtio.h
    @@ -69,6 +69,7 @@ struct virtio_device {
    /* Note that this is a Linux set_bit-style bitmap. */
    unsigned long features[1];
    void *priv;
    + unsigned int indirect_thresh;
    };

    #define dev_to_virtio(dev) container_of(dev, struct virtio_device, dev)
    diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
    index 35b8911..fc93962 100644
    --- a/net/9p/trans_virtio.c
    +++ b/net/9p/trans_virtio.c
    @@ -52,6 +52,9 @@

    #define VIRTQUEUE_NUM 128

    +static unsigned int indirect_thresh;
    +module_param(indirect_thresh, uint, S_IRUGO);
    +
    /* a single mutex to manage channel initialization and attachment */
    static DEFINE_MUTEX(virtio_9p_lock);
    static DECLARE_WAIT_QUEUE_HEAD(vp_wq);
    @@ -501,6 +504,7 @@ static int p9_virtio_probe(struct virtio_device *vdev)
    chan->vdev = vdev;

    /* We expect one virtqueue, for requests. */
    + vdev->indirect_thresh = indirect_thresh;
    chan->vq = virtio_find_single_vq(vdev, req_done, "requests");
    if (IS_ERR(chan->vq)) {
    err = PTR_ERR(chan->vq);
    --
    1.7.12


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