Re: [PATCH v14 3/5] virtio-balloon: VIRTIO_BALLOON_F_SG

[Michael S. Tsirkin]

On Thu, Aug 17, 2017 at 11:26:54AM +0800, Wei Wang wrote:
> Add a new feature, VIRTIO_BALLOON_F_SG, which enables the transfer
> of balloon (i.e. inflated/deflated) pages using scatter-gather lists
> to the host.
> 
> The implementation of the previous virtio-balloon is not very
> efficient, because the balloon pages are transferred to the
> host one by one. Here is the breakdown of the time in percentage
> spent on each step of the balloon inflating process (inflating
> 7GB of an 8GB idle guest).
> 
> 1) allocating pages (6.5%)
> 2) sending PFNs to host (68.3%)
> 3) address translation (6.1%)
> 4) madvise (19%)
> 
> It takes about 4126ms for the inflating process to complete.
> The above profiling shows that the bottlenecks are stage 2)
> and stage 4).
> 
> This patch optimizes step 2) by transferring pages to the host in
> sgs. An sg describes a chunk of guest physically continuous pages.
> With this mechanism, step 4) can also be optimized by doing address
> translation and madvise() in chunks rather than page by page.
> 
> With this new feature, the above ballooning process takes ~541ms
> resulting in an improvement of ~87%.
> 
> TODO: optimize stage 1) by allocating/freeing a chunk of pages
> instead of a single page each time.
> 
> Signed-off-by: Wei Wang <wei.w.wang@xxxxxxxxx>
> Signed-off-by: Liang Li <liang.z.li@xxxxxxxxx>
> Suggested-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
>  drivers/virtio/virtio_balloon.c     | 157 ++++++++++++++++++++++++++++++++----
>  include/uapi/linux/virtio_balloon.h |   1 +
>  2 files changed, 141 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index f0b3a0b..72041b4 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -32,6 +32,7 @@
>  #include <linux/mm.h>
>  #include <linux/mount.h>
>  #include <linux/magic.h>
> +#include <linux/xbitmap.h>
>  
>  /*
>   * Balloon device works in 4K page units.  So each page is pointed to by
> @@ -79,6 +80,9 @@ struct virtio_balloon {
>  	/* Synchronize access/update to this struct virtio_balloon elements */
>  	struct mutex balloon_lock;
>  
> +	/* The xbitmap used to record ballooned pages */
> +	struct xb page_xb;
> +
>  	/* The array of pfns we tell the Host about. */
>  	unsigned int num_pfns;
>  	__virtio32 pfns[VIRTIO_BALLOON_ARRAY_PFNS_MAX];
> @@ -141,13 +145,98 @@ static void set_page_pfns(struct virtio_balloon *vb,
>  					  page_to_balloon_pfn(page) + i);
>  }
>  
> +static int add_one_sg(struct virtqueue *vq, void *addr, uint32_t size)
> +{
> +	struct scatterlist sg;
> +
> +	sg_init_one(&sg, addr, size);
> +	return virtqueue_add_inbuf(vq, &sg, 1, vq, GFP_KERNEL);
> +}
> +
> +static void send_balloon_page_sg(struct virtio_balloon *vb,
> +				 struct virtqueue *vq,
> +				 void *addr,
> +				 uint32_t size)
> +{
> +	unsigned int len;
> +	int ret;
> +
> +	do {
> +		ret = add_one_sg(vq, addr, size);
> +		virtqueue_kick(vq);
> +		wait_event(vb->acked, virtqueue_get_buf(vq, &len));
> +		/*
> +		 * It is uncommon to see the vq is full, because the sg is sent
> +		 * one by one and the device is able to handle it in time. But
> +		 * if that happens, we go back to retry after an entry gets
> +		 * released.
> +		 */

Why send one by one though? Why not batch some s/gs and wait for all
of them to be completed? If memory if fragmented, waiting every time is
worse than what we have now (VIRTIO_BALLOON_ARRAY_PFNS_MAX at a time).

> +	} while (unlikely(ret == -ENOSPC));
> +}
> +
> +/*
> + * Send balloon pages in sgs to host. The balloon pages are recorded in the
> + * page xbitmap. Each bit in the bitmap corresponds to a page of PAGE_SIZE.
> + * The page xbitmap is searched for continuous "1" bits, which correspond
> + * to continuous pages, to chunk into sgs.
> + *
> + * @page_xb_start and @page_xb_end form the range of bits in the xbitmap that
> + * need to be searched.
> + */
> +static void tell_host_sgs(struct virtio_balloon *vb,
> +			  struct virtqueue *vq,
> +			  unsigned long page_xb_start,
> +			  unsigned long page_xb_end)
> +{
> +	unsigned long sg_pfn_start, sg_pfn_end;
> +	void *sg_addr;
> +	uint32_t sg_len, sg_max_len = round_down(UINT_MAX, PAGE_SIZE);
> +
> +	sg_pfn_start = page_xb_start;
> +	while (sg_pfn_start < page_xb_end) {
> +		sg_pfn_start = xb_find_next_bit(&vb->page_xb, sg_pfn_start,
> +						page_xb_end, 1);
> +		if (sg_pfn_start == page_xb_end + 1)
> +			break;
> +		sg_pfn_end = xb_find_next_bit(&vb->page_xb, sg_pfn_start + 1,
> +					      page_xb_end, 0);
> +		sg_addr = (void *)pfn_to_kaddr(sg_pfn_start);
> +		sg_len = (sg_pfn_end - sg_pfn_start) << PAGE_SHIFT;
> +		while (sg_len > sg_max_len) {
> +			send_balloon_page_sg(vb, vq, sg_addr, sg_max_len);
> +			sg_addr += sg_max_len;
> +			sg_len -= sg_max_len;
> +		}
> +		send_balloon_page_sg(vb, vq, sg_addr, sg_len);
> +		xb_zero(&vb->page_xb, sg_pfn_start, sg_pfn_end);
> +		sg_pfn_start = sg_pfn_end + 1;
> +	}
> +}
> +
> +static inline void xb_set_page(struct virtio_balloon *vb,
> +			       struct page *page,
> +			       unsigned long *pfn_min,
> +			       unsigned long *pfn_max)
> +{
> +	unsigned long pfn = page_to_pfn(page);
> +
> +	*pfn_min = min(pfn, *pfn_min);
> +	*pfn_max = max(pfn, *pfn_max);
> +	xb_preload(GFP_KERNEL);
> +	xb_set_bit(&vb->page_xb, pfn);
> +	xb_preload_end();
> +}
> +
>  static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
>  {
>  	struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
>  	unsigned num_allocated_pages;
> +	bool use_sg = virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_SG);
> +	unsigned long pfn_max = 0, pfn_min = ULONG_MAX;
>  
>  	/* We can only do one array worth at a time. */
> -	num = min(num, ARRAY_SIZE(vb->pfns));
> +	if (!use_sg)
> +		num = min(num, ARRAY_SIZE(vb->pfns));
>  
>  	mutex_lock(&vb->balloon_lock);
>  	for (vb->num_pfns = 0; vb->num_pfns < num;
> @@ -162,7 +251,12 @@ static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
>  			msleep(200);
>  			break;
>  		}
> -		set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
> +
> +		if (use_sg)
> +			xb_set_page(vb, page, &pfn_min, &pfn_max);
> +		else
> +			set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
> +
>  		vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
>  		if (!virtio_has_feature(vb->vdev,
>  					VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> @@ -171,8 +265,12 @@ static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
>  
>  	num_allocated_pages = vb->num_pfns;
>  	/* Did we get any? */
> -	if (vb->num_pfns != 0)
> -		tell_host(vb, vb->inflate_vq);
> +	if (vb->num_pfns) {
> +		if (use_sg)
> +			tell_host_sgs(vb, vb->inflate_vq, pfn_min, pfn_max);
> +		else
> +			tell_host(vb, vb->inflate_vq);
> +	}
>  	mutex_unlock(&vb->balloon_lock);
>  
>  	return num_allocated_pages;
> @@ -198,9 +296,12 @@ static unsigned leak_balloon(struct virtio_balloon *vb, size_t num)
>  	struct page *page;
>  	struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
>  	LIST_HEAD(pages);
> +	bool use_sg = virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_SG);
> +	unsigned long pfn_max = 0, pfn_min = ULONG_MAX;
>  
> -	/* We can only do one array worth at a time. */
> -	num = min(num, ARRAY_SIZE(vb->pfns));
> +	/* Traditionally, we can only do one array worth at a time. */
> +	if (!use_sg)
> +		num = min(num, ARRAY_SIZE(vb->pfns));
>  
>  	mutex_lock(&vb->balloon_lock);
>  	/* We can't release more pages than taken */
> @@ -210,7 +311,11 @@ static unsigned leak_balloon(struct virtio_balloon *vb, size_t num)
>  		page = balloon_page_dequeue(vb_dev_info);
>  		if (!page)
>  			break;
> -		set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
> +		if (use_sg)
> +			xb_set_page(vb, page, &pfn_min, &pfn_max);
> +		else
> +			set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
> +
>  		list_add(&page->lru, &pages);
>  		vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
>  	}
> @@ -221,8 +326,12 @@ static unsigned leak_balloon(struct virtio_balloon *vb, size_t num)
>  	 * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
>  	 * is true, we *have* to do it in this order
>  	 */
> -	if (vb->num_pfns != 0)
> -		tell_host(vb, vb->deflate_vq);
> +	if (vb->num_pfns) {
> +		if (use_sg)
> +			tell_host_sgs(vb, vb->deflate_vq, pfn_min, pfn_max);
> +		else
> +			tell_host(vb, vb->deflate_vq);
> +	}
>  	release_pages_balloon(vb, &pages);
>  	mutex_unlock(&vb->balloon_lock);
>  	return num_freed_pages;
> @@ -441,6 +550,7 @@ static int init_vqs(struct virtio_balloon *vb)
>  }
>  
>  #ifdef CONFIG_BALLOON_COMPACTION
> +
>  /*
>   * virtballoon_migratepage - perform the balloon page migration on behalf of
>   *			     a compation thread.     (called under page lock)
> @@ -464,6 +574,7 @@ static int virtballoon_migratepage(struct balloon_dev_info *vb_dev_info,
>  {
>  	struct virtio_balloon *vb = container_of(vb_dev_info,
>  			struct virtio_balloon, vb_dev_info);
> +	bool use_sg = virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_SG);
>  	unsigned long flags;
>  
>  	/*
> @@ -485,16 +596,24 @@ static int virtballoon_migratepage(struct balloon_dev_info *vb_dev_info,
>  	vb_dev_info->isolated_pages--;
>  	__count_vm_event(BALLOON_MIGRATE);
>  	spin_unlock_irqrestore(&vb_dev_info->pages_lock, flags);
> -	vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
> -	set_page_pfns(vb, vb->pfns, newpage);
> -	tell_host(vb, vb->inflate_vq);
> -
> +	if (use_sg) {
> +		send_balloon_page_sg(vb, vb->inflate_vq, page_address(newpage),
> +				     PAGE_SIZE);
> +	} else {
> +		vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
> +		set_page_pfns(vb, vb->pfns, newpage);
> +		tell_host(vb, vb->inflate_vq);
> +	}
>  	/* balloon's page migration 2nd step -- deflate "page" */
>  	balloon_page_delete(page);
> -	vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
> -	set_page_pfns(vb, vb->pfns, page);
> -	tell_host(vb, vb->deflate_vq);
> -
> +	if (use_sg) {
> +		send_balloon_page_sg(vb, vb->deflate_vq, page_address(page),
> +				     PAGE_SIZE);
> +	} else {
> +		vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
> +		set_page_pfns(vb, vb->pfns, page);
> +		tell_host(vb, vb->deflate_vq);
> +	}
>  	mutex_unlock(&vb->balloon_lock);
>  
>  	put_page(page); /* balloon reference */
> @@ -553,6 +672,9 @@ static int virtballoon_probe(struct virtio_device *vdev)
>  	if (err)
>  		goto out_free_vb;
>  
> +	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_SG))
> +		xb_init(&vb->page_xb);
> +
>  	vb->nb.notifier_call = virtballoon_oom_notify;
>  	vb->nb.priority = VIRTBALLOON_OOM_NOTIFY_PRIORITY;
>  	err = register_oom_notifier(&vb->nb);
> @@ -669,6 +791,7 @@ static unsigned int features[] = {
>  	VIRTIO_BALLOON_F_MUST_TELL_HOST,
>  	VIRTIO_BALLOON_F_STATS_VQ,
>  	VIRTIO_BALLOON_F_DEFLATE_ON_OOM,
> +	VIRTIO_BALLOON_F_SG,
>  };
>  
>  static struct virtio_driver virtio_balloon_driver = {
> diff --git a/include/uapi/linux/virtio_balloon.h b/include/uapi/linux/virtio_balloon.h
> index 343d7dd..37780a7 100644
> --- a/include/uapi/linux/virtio_balloon.h
> +++ b/include/uapi/linux/virtio_balloon.h
> @@ -34,6 +34,7 @@
>  #define VIRTIO_BALLOON_F_MUST_TELL_HOST	0 /* Tell before reclaiming pages */
>  #define VIRTIO_BALLOON_F_STATS_VQ	1 /* Memory Stats virtqueue */
>  #define VIRTIO_BALLOON_F_DEFLATE_ON_OOM	2 /* Deflate balloon on OOM */
> +#define VIRTIO_BALLOON_F_SG		3 /* Use sg instead of PFN lists */
>  
>  /* Size of a PFN in the balloon interface. */
>  #define VIRTIO_BALLOON_PFN_SHIFT 12
> -- 
> 2.7.4