Re: [Qemu-devel] [PATCH v18 07/10] virtio-balloon: VIRTIO_BALLOON_F_SG

[Michael S. Tsirkin]

On Wed, Nov 29, 2017 at 09:55:23PM +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. A scatter-gather list is described by a vring desc.
> 
> The implementation of the previous virtio-balloon is not very efficient,
> because the balloon pages are transferred to the host by one array each
> time. 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 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 ~440ms resulting
> in an improvement of ~89%.
> 
> TODO: optimize stage 1) by allocating/freeing a chunk of pages instead of
> a single page each time.
> 
> Signed-off-by: Wei Wang <address@hidden>
> Signed-off-by: Liang Li <address@hidden>
> Suggested-by: Michael S. Tsirkin <address@hidden>
> Cc: Tetsuo Handa <address@hidden>
> ---
>  drivers/virtio/virtio_balloon.c     | 230 
> +++++++++++++++++++++++++++++++++---
>  include/uapi/linux/virtio_balloon.h |   1 +
>  2 files changed, 212 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 7960746..2c21c5a 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -32,6 +32,8 @@
>  #include <linux/mm.h>
>  #include <linux/mount.h>
>  #include <linux/magic.h>
> +#include <linux/xbitmap.h>
> +#include <asm/page.h>
>  
>  /*
>   * Balloon device works in 4K page units.  So each page is pointed to by
> @@ -79,6 +81,9 @@ struct virtio_balloon {
>       /* Synchronize access/update to this struct virtio_balloon elements */
>       struct mutex balloon_lock;
>  
> +     /* The xbitmap used to record balloon 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];
> @@ -141,15 +146,121 @@ static void set_page_pfns(struct virtio_balloon *vb,
>                                         page_to_balloon_pfn(page) + i);
>  }
>  
> +static void kick_and_wait(struct virtqueue *vq, wait_queue_head_t wq_head)
> +{
> +     unsigned int len;
> +
> +     virtqueue_kick(vq);
> +     wait_event(wq_head, virtqueue_get_buf(vq, &len));
> +}
> +
> +static void send_one_desc(struct virtio_balloon *vb,
> +                       struct virtqueue *vq,
> +                       uint64_t addr,
> +                       uint32_t len,
> +                       bool inbuf,
> +                       bool batch)
> +{
> +     int err;
> +     unsigned int size;
> +
> +     /* Detach all the used buffers from the vq */
> +     while (virtqueue_get_buf(vq, &size))
> +             ;
> +
> +     err = virtqueue_add_one_desc(vq, addr, len, inbuf, vq);
> +     /*
> +      * This is expected to never fail: there is always at least 1 entry
> +      * available on the vq, because when the vq is full the worker thread
> +      * that adds the desc will be put into sleep until at least 1 entry is
> +      * available to use.
> +      */
> +     BUG_ON(err);
> +
> +     /* If batching is requested, we batch till the vq is full */
> +     if (!batch || !vq->num_free)
> +             kick_and_wait(vq, vb->acked);
> +}
> +

This internal kick complicates callers. I suggest that instead,
you move this to callers, just return a "kick required" boolean.
This way callers do not need to play with num_free at all.

> +/*
> + * 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 pfn_start, pfn_end;
> +     uint64_t addr;
> +     uint32_t len, max_len = round_down(UINT_MAX, PAGE_SIZE);
> +
> +     pfn_start = page_xb_start;
> +     while (pfn_start < page_xb_end) {
> +             pfn_start = xb_find_next_set_bit(&vb->page_xb, pfn_start,
> +                                              page_xb_end);
> +             if (pfn_start == page_xb_end + 1)
> +                     break;
> +             pfn_end = xb_find_next_zero_bit(&vb->page_xb,
> +                                             pfn_start + 1,
> +                                             page_xb_end);
> +             addr = pfn_start << PAGE_SHIFT;
> +             len = (pfn_end - pfn_start) << PAGE_SHIFT;

This assugnment can overflow. Next line compares with UINT_MAX but by
that time it is too late.  I think you should do all math in 64 bit to
avoid surprises, then truncate to max_len and then it's safe to assign
to sg.

> +             while (len > max_len) {
> +                     send_one_desc(vb, vq, addr, max_len, true, true);
> +                     addr += max_len;
> +                     len -= max_len;
> +             }
> +             send_one_desc(vb, vq, addr, len, true, true);
> +             pfn_start = pfn_end + 1;
> +     }
> +
> +     /*
> +      * The last few desc-s may not reach the batch size, but need a kick to
> +      * notify the device to handle them.
> +      */
> +     if (vq->num_free != virtqueue_get_vring_size(vq))
> +             kick_and_wait(vq, vb->acked);

the actual trigger for kick is if we did not kick after
the last send_one_desc. If kick is moved out of send_one_desc,
you can test for that explicitly.

> +
> +     xb_clear_bit_range(&vb->page_xb, page_xb_start, page_xb_end);
> +}
> +
> +static inline int 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);
> +     int ret;
> +
> +     *pfn_min = min(pfn, *pfn_min);
> +     *pfn_max = max(pfn, *pfn_max);
> +
> +     do {
> +             ret = xb_preload_and_set_bit(&vb->page_xb, pfn,
> +                                          GFP_NOWAIT | __GFP_NOWARN);
> +     } while (unlikely(ret == -EAGAIN));

what exactly does this loop do? Does this wait
forever until there is some free memory? why GFP_NOWAIT?

> +
> +     return ret;
> +}
> +
>  static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
>  {
>       unsigned num_allocated_pages;
>       unsigned num_pfns;
>       struct page *page;
>       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));
> +     if (!use_sg)
> +             num = min(num, ARRAY_SIZE(vb->pfns));
>  
>       for (num_pfns = 0; num_pfns < num;
>            num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
> @@ -172,11 +283,18 @@ static unsigned fill_balloon(struct virtio_balloon *vb, 
> size_t num)
>       vb->num_pfns = 0;
>  
>       while ((page = balloon_page_pop(&pages))) {
> +             if (use_sg) {
> +                     if (xb_set_page(vb, page, &pfn_min, &pfn_max) < 0) {
> +                             __free_page(page);
> +                             break;
> +                     }
> +             } else {
> +                     set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
> +             }
> +
>               balloon_page_enqueue(&vb->vb_dev_info, page);
>  
>               vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE;
> -
> -             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))
> @@ -185,8 +303,12 @@ 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;
> @@ -212,9 +334,12 @@ 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 */
> @@ -224,7 +349,14 @@ 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) {
> +                     if (xb_set_page(vb, page, &pfn_min, &pfn_max) < 0) {
> +                             balloon_page_enqueue(&vb->vb_dev_info, page);
> +                             break;
> +                     }
> +             } else {
> +                     set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
> +             }
>               list_add(&page->lru, &pages);
>               vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
>       }
> @@ -235,13 +367,55 @@ 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;
>  }
>  
> +/*
> + * The regular leak_balloon() with VIRTIO_BALLOON_F_SG needs memory 
> allocation
> + * for xbitmap, which is not suitable for the oom case. This function does 
> not
> + * use xbitmap to chunk pages, so it can be used by oom notifier to deflate
> + * pages when VIRTIO_BALLOON_F_SG is negotiated.
> + */

I guess we can live with this for now.

Two things to consider
- adding support for pre-allocating indirect buffers
- sorting the internal page queue (how?)


> +static unsigned int leak_balloon_sg_oom(struct virtio_balloon *vb)
> +{
> +     unsigned int n;
> +     struct page *page;
> +     struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
> +     struct virtqueue *vq = vb->deflate_vq;
> +     LIST_HEAD(pages);
> +
> +     mutex_lock(&vb->balloon_lock);
> +     for (n = 0; n < oom_pages; n++) {
> +             page = balloon_page_dequeue(vb_dev_info);
> +             if (!page)
> +                     break;
> +
> +             list_add(&page->lru, &pages);
> +             vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
> +             send_one_desc(vb, vq, virt_to_phys(page_address(page)),
> +                           PAGE_SIZE, true, true);
> +             release_pages_balloon(vb, &pages);
> +     }
> +
> +     /*
> +      * The last few sgs may not reach the batch size, but need a kick to
> +      * notify the device to handle them.
> +      */
> +     if (vq->num_free != virtqueue_get_vring_size(vq))
> +             kick_and_wait(vq, vb->acked);
> +     mutex_unlock(&vb->balloon_lock);
> +
> +     return n;
> +}
> +
>  static inline void update_stat(struct virtio_balloon *vb, int idx,
>                              u16 tag, u64 val)
>  {
> @@ -381,7 +555,10 @@ static int virtballoon_oom_notify(struct notifier_block 
> *self,
>               return NOTIFY_OK;
>  
>       freed = parm;
> -     num_freed_pages = leak_balloon(vb, oom_pages);
> +     if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_SG))
> +             num_freed_pages = leak_balloon_sg_oom(vb);
> +     else
> +             num_freed_pages = leak_balloon(vb, oom_pages);
>       update_balloon_size(vb);
>       *freed += num_freed_pages;
>  
> @@ -478,6 +655,7 @@ 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;
>  
>       /*
> @@ -499,16 +677,26 @@ 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_one_desc(vb, vb->inflate_vq,
> +                           virt_to_phys(page_address(newpage)),
> +                           PAGE_SIZE, true, false);
> +     } 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_one_desc(vb, vb->inflate_vq,
> +                           virt_to_phys(page_address(page)),
> +                           PAGE_SIZE, true, false);
> +     } 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 */
> @@ -567,6 +755,9 @@ 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);
> @@ -683,6 +874,7 @@ 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