On Sun, 31 Oct 2021 10:46:12 -0400, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> On Thu, Oct 28, 2021 at 06:49:17PM +0800, Xuan Zhuo wrote:
> > In the case of using indirect, indirect desc must be allocated and
> > released each time, which increases a lot of cpu overhead.
> >
> > Here, a cache is added for indirect. If the number of indirect desc to be
> > applied for is less than VIRT_QUEUE_CACHE_DESC_NUM, the desc array with
> > the size of VIRT_QUEUE_CACHE_DESC_NUM is fixed and cached for reuse.
> >
> > Signed-off-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
>
> What bothers me here is what happens if cache gets
> filled on one numa node, then used on another?
Well, this is a good question, I didn't think about it before.
In this way, I feel that using kmem_cache_alloc's series of functions is a good
solution. But when I tested it before, there was no improvement. I want to study
the reasons for this, and hope to solve this problem.
In addition, regarding the use of the kmem_cache_alloc_bulk function you
mentioned, I will also try to see it.
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index dd95dfd85e98..0ebcd4f12d3b 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -117,6 +117,15 @@ struct vring_virtqueue {
> > /* Hint for event idx: already triggered no need to disable. */
> > bool event_triggered;
> >
> > + /* desc cache threshold
> > + * 0 - disable desc cache
> > + * > 0 - enable desc cache. As the threshold of the desc cache.
> > + */
> > + u32 desc_cache_thr;
>
> not really descriptive. also pls eschew abbreviation.
>
> > +
> > + /* desc cache chain */
> > + struct list_head desc_cache;
>
> hmm this puts extra pressure on cache. you never need to drop
> things in the middle. llist_head would be better I
> think ... no?
I tried to use llist_head before, but I found that it is a less-locked
implementation with some atomic operations. Since the cache here is exclusive to
vq, I feel that there is no need to use atomic operations. I replaced it with
list_head.
I don't know what you mean by "things in the middel".
>
>
> > +
> > union {
> > /* Available for split ring */
> > struct {
> > @@ -423,7 +432,53 @@ static unsigned int vring_unmap_one_split(const struct vring_virtqueue *vq,
> > return extra[i].next;
> > }
> >
> > -static struct vring_desc *alloc_indirect_split(struct virtqueue *_vq,
> > +static void desc_cache_free(struct list_head *head)
> > +{
> > + struct list_head *n, *pos;
> > +
> > + BUILD_BUG_ON(sizeof(struct list_head) > sizeof(struct vring_desc));
> > + BUILD_BUG_ON(sizeof(struct list_head) > sizeof(struct vring_packed_desc));
> > +
> > + list_for_each_prev_safe(pos, n, head)
> > + kfree(pos);
> > +}
> > +
> > +static void __desc_cache_put(struct vring_virtqueue *vq,
> > + struct list_head *node, int n)
> > +{
> > + if (n <= vq->desc_cache_thr)
> > + list_add(node, &vq->desc_cache);
> > + else
> > + kfree(node);
>
> this bothers me. Do we really need a full VQ's worth of
> indirect descriptors? Can't we set a limit on how many
> are used?
>
I think, I don't quite understand what you mean, I guess you mean, do multiple
vqs share a cache?
Then do we limit the total size of the shared space?
When the user calls virtqueue_add, if the number of sg used is greater than 1,
indirect desc will be used. If the number of sg is less than or equal to
desc_cache_thr, cache is used. When it is greater than
desc_cache_thr, kmalloc/kfree is used directly.
Because each item in the cache is a fixed number(desc_cache_thr) desc.
>
> > +}
> > +
> > +#define desc_cache_put(vq, desc, n) \
> > + __desc_cache_put(vq, (struct list_head *)desc, n)
>
> replace with an inline function pls. in fact we dont need
> __desc_cache_put at all.
>
>
> > +
> > +static void *desc_cache_get(struct vring_virtqueue *vq,
> > + int size, int n, gfp_t gfp)
> > +{
> > + struct list_head *node;
> > +
> > + if (n > vq->desc_cache_thr)
> > + return kmalloc_array(n, size, gfp);
> > +
> > + if (!list_empty(&vq->desc_cache)) {
> > + node = vq->desc_cache.next;
> > + list_del(node);
> > + return node;
> > + }
> > +
> > + return kmalloc_array(vq->desc_cache_thr, size, gfp);
> > +}
> > +
> > +#define _desc_cache_get(vq, n, gfp, tp) \
> > + ((tp *)desc_cache_get(vq, (sizeof(tp)), n, gfp))
> > +
> > +#define desc_cache_get_split(vq, n, gfp) \
> > + _desc_cache_get(vq, n, gfp, struct vring_desc)
> > +
>
> same thing here.
>
> > +static struct vring_desc *alloc_indirect_split(struct vring_virtqueue *vq,
> > unsigned int total_sg,
> > gfp_t gfp)
> > {
> > @@ -437,12 +492,12 @@ static struct vring_desc *alloc_indirect_split(struct virtqueue *_vq,
> > */
> > gfp &= ~__GFP_HIGHMEM;
> >
> > - desc = kmalloc_array(total_sg, sizeof(struct vring_desc), gfp);
> > + desc = desc_cache_get_split(vq, total_sg, gfp);
> > if (!desc)
> > return NULL;
> >
> > for (i = 0; i < total_sg; i++)
> > - desc[i].next = cpu_to_virtio16(_vq->vdev, i + 1);
> > + desc[i].next = cpu_to_virtio16(vq->vq.vdev, i + 1);
> > return desc;
> > }
> >
> > @@ -508,7 +563,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> > head = vq->free_head;
> >
> > if (virtqueue_use_indirect(_vq, total_sg))
> > - desc = alloc_indirect_split(_vq, total_sg, gfp);
> > + desc = alloc_indirect_split(vq, total_sg, gfp);
> > else {
> > desc = NULL;
> > WARN_ON_ONCE(total_sg > vq->split.vring.num && !vq->indirect);
> > @@ -652,7 +707,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> > }
> >
> > if (indirect)
> > - kfree(desc);
> > + desc_cache_put(vq, desc, total_sg);
> >
> > END_USE(vq);
> > return -ENOMEM;
> > @@ -717,7 +772,7 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
> > if (vq->indirect) {
> > struct vring_desc *indir_desc =
> > vq->split.desc_state[head].indir_desc;
> > - u32 len;
> > + u32 len, n;
> >
> > /* Free the indirect table, if any, now that it's unmapped. */
> > if (!indir_desc)
> > @@ -729,10 +784,12 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
> > VRING_DESC_F_INDIRECT));
> > BUG_ON(len == 0 || len % sizeof(struct vring_desc));
> >
> > - for (j = 0; j < len / sizeof(struct vring_desc); j++)
> > + n = len / sizeof(struct vring_desc);
> > +
> > + for (j = 0; j < n; j++)
> > vring_unmap_one_split_indirect(vq, &indir_desc[j]);
> >
> > - kfree(indir_desc);
> > + desc_cache_put(vq, indir_desc, n);
> > vq->split.desc_state[head].indir_desc = NULL;
> > } else if (ctx) {
> > *ctx = vq->split.desc_state[head].indir_desc;
> > @@ -2199,6 +2256,9 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
> > vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC) &&
> > !context;
> > vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
> > + vq->desc_cache_thr = vdev->desc_cache_thr;
> > +
> > + INIT_LIST_HEAD(&vq->desc_cache);
> >
> > if (virtio_has_feature(vdev, VIRTIO_F_ORDER_PLATFORM))
> > vq->weak_barriers = false;
>
> So e.g. for rx, we are wasting memory since indirect isn't used.
Yes, thank you for pointing this out. I ignore this. So pls ignore v4 with
pre-alloc cache.
Thanks.
>
>
> > @@ -2329,6 +2389,7 @@ void vring_del_virtqueue(struct virtqueue *_vq)
> > if (!vq->packed_ring) {
> > kfree(vq->split.desc_state);
> > kfree(vq->split.desc_extra);
> > + desc_cache_free(&vq->desc_cache);
> > }
> > kfree(vq);
> > }
> > diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> > index 41edbc01ffa4..bda6f9853e97 100644
> > --- a/include/linux/virtio.h
> > +++ b/include/linux/virtio.h
> > @@ -118,6 +118,7 @@ struct virtio_device {
> > struct list_head vqs;
> > u64 features;
> > void *priv;
> > + u32 desc_cache_thr;
> > };
> >
> > static inline struct virtio_device *dev_to_virtio(struct device *_dev)
> > @@ -130,6 +131,19 @@ int register_virtio_device(struct virtio_device *dev);
> > void unregister_virtio_device(struct virtio_device *dev);
> > bool is_virtio_device(struct device *dev);
> >
> > +/**
> > + * virtio_set_desc_cache - set virtio ring desc cache threshold
> > + *
> > + * virtio will cache the allocated indirect desc.
> > + *
> > + * This function must be called before find_vqs.
> > + *
> > + * @thr:
> > + * 0 - disable desc cache
> > + * > 0 - enable desc cache. As the threshold of the desc cache.
> > + */
> > +void virtio_set_desc_cache(struct virtio_device *dev, u32 thr);
> > +
> > void virtio_break_device(struct virtio_device *dev);
> >
> > void virtio_config_changed(struct virtio_device *dev);
> > --
> > 2.31.0
>
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