[Qemu-devel] [PATCH RFC kernel] balloon: speed up inflating/deflating process

[Liang Li]

The implementation of the current virtio-balloon is not very efficient,
Bellow is test result of time spends on inflating the balloon to 3GB of
a 4GB idle guest:

a. allocating pages (6.5%, 103ms)
b. sending PFNs to host (68.3%, 787ms)
c. address translation (6.1%, 96ms)
d. madvise (19%, 300ms)

It takes about 1577ms for the whole inflating process to complete. The
test shows that the bottle neck is the stage b and stage d.

If using a bitmap to send the page info instead of the PFNs, we can
reduce the overhead spends on stage b quite a lot. Furthermore, it's
possible to do the address translation and do the madvise with a bulk
of pages, instead of the current page per page way, so the overhead of
stage c and stage d can also be reduced a lot.

This patch is the kernel side implementation which is intended to speed
up the inflating & deflating process by adding a new feature to the
virtio-balloon device. And now, inflating the balloon to 3GB of a 4GB
idle guest only takes 175ms, it's about 9 times as fast as before.

TODO: optimize stage a by allocating/freeing a chunk of pages instead
of a single page at a time.

Signed-off-by: Liang Li <address@hidden>
---
 drivers/virtio/virtio_balloon.c     | 199 ++++++++++++++++++++++++++++++++++--
 include/uapi/linux/virtio_balloon.h |   1 +
 mm/page_alloc.c                     |   6 ++
 3 files changed, 198 insertions(+), 8 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 7b6d74f..5330b6f 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -45,6 +45,8 @@ static int oom_pages = OOM_VBALLOON_DEFAULT_PAGES;
 module_param(oom_pages, int, S_IRUSR | S_IWUSR);
 MODULE_PARM_DESC(oom_pages, "pages to free on OOM");
 
+extern unsigned long get_max_pfn(void);
+
 struct virtio_balloon {
        struct virtio_device *vdev;
        struct virtqueue *inflate_vq, *deflate_vq, *stats_vq;
@@ -62,6 +64,9 @@ struct virtio_balloon {
 
        /* Number of balloon pages we've told the Host we're not using. */
        unsigned int num_pages;
+       unsigned long *page_bitmap;
+       unsigned long start_pfn, end_pfn;
+       unsigned long bmap_len;
        /*
         * The pages we've told the Host we're not using are enqueued
         * at vb_dev_info->pages list.
@@ -111,15 +116,66 @@ static void balloon_ack(struct virtqueue *vq)
        wake_up(&vb->acked);
 }
 
+static int balloon_page_bitmap_init(struct virtio_balloon *vb)
+{
+       unsigned long max_pfn, bmap_bytes;
+
+       max_pfn = get_max_pfn();
+       bmap_bytes = ALIGN(max_pfn, BITS_PER_LONG) / BITS_PER_BYTE;
+       if (!vb->page_bitmap)
+               vb->page_bitmap = kzalloc(bmap_bytes, GFP_KERNEL);
+       else {
+               if (bmap_bytes <= vb->bmap_len)
+                       memset(vb->page_bitmap, 0, bmap_bytes);
+               else {
+                       kfree(vb->page_bitmap);
+                       vb->page_bitmap = kzalloc(bmap_bytes, GFP_KERNEL);
+               }
+       }
+       if (!vb->page_bitmap) {
+               dev_err(&vb->vdev->dev, "%s failure: allocate page bitmap\n",
+                        __func__);
+               return -ENOMEM;
+       }
+       vb->bmap_len = bmap_bytes;
+       vb->start_pfn = max_pfn;
+       vb->end_pfn = 0;
+
+       return 0;
+}
+
 static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
 {
-       struct scatterlist sg;
        unsigned int len;
 
-       sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns);
+       if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_PAGE_BITMAP)) {
+               u32 page_shift = PAGE_SHIFT;
+               unsigned long start_pfn, end_pfn, flags = 0, bmap_len;
+               struct scatterlist sg[5];
+
+               start_pfn = rounddown(vb->start_pfn, BITS_PER_LONG);
+               end_pfn = roundup(vb->end_pfn, BITS_PER_LONG);
+               bmap_len = (end_pfn - start_pfn) / BITS_PER_LONG * sizeof(long);
+
+               sg_init_table(sg, 5);
+               sg_set_buf(&sg[0], &flags, sizeof(flags));
+               sg_set_buf(&sg[1], &start_pfn, sizeof(start_pfn));
+               sg_set_buf(&sg[2], &page_shift, sizeof(page_shift));
+               sg_set_buf(&sg[3], &bmap_len, sizeof(bmap_len));
+               sg_set_buf(&sg[4], vb->page_bitmap +
+                                (start_pfn / BITS_PER_LONG), bmap_len);
+               virtqueue_add_outbuf(vq, sg, 5, vb, GFP_KERNEL);
+
+       } else {
+               struct scatterlist sg;
+
+               sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns);
+               /* We should always be able to add one buffer to an
+               * empty queue.
+               */
+               virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
+       }
 
-       /* We should always be able to add one buffer to an empty queue. */
-       virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
        virtqueue_kick(vq);
 
        /* When host has read buffer, this completes via balloon_ack */
@@ -137,7 +193,21 @@ static void set_page_pfns(u32 pfns[], struct page *page)
                pfns[i] = page_to_balloon_pfn(page) + i;
 }
 
-static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
+static void set_page_bitmap(struct virtio_balloon *vb, struct page *page)
+{
+       unsigned int i;
+       unsigned long *bitmap = vb->page_bitmap;
+       unsigned long balloon_pfn = page_to_balloon_pfn(page);
+
+       for (i = 0; i < VIRTIO_BALLOON_PAGES_PER_PAGE; i++)
+               set_bit(balloon_pfn + i, bitmap);
+       if (balloon_pfn < vb->start_pfn)
+               vb->start_pfn = balloon_pfn;
+       if (balloon_pfn > vb->end_pfn)
+               vb->end_pfn = balloon_pfn;
+}
+
+static unsigned fill_balloon_pfns(struct virtio_balloon *vb, size_t num)
 {
        struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
        unsigned num_allocated_pages;
@@ -174,7 +244,104 @@ static unsigned fill_balloon(struct virtio_balloon *vb, 
size_t num)
        return num_allocated_pages;
 }
 
-static void release_pages_balloon(struct virtio_balloon *vb)
+static long fill_balloon_bitmap(struct virtio_balloon *vb, size_t num)
+{
+       struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
+       long num_allocated_pages = 0;
+
+       if (balloon_page_bitmap_init(vb) < 0)
+               return num;
+
+       mutex_lock(&vb->balloon_lock);
+       for (vb->num_pfns = 0; vb->num_pfns < num;
+            vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
+               struct page *page = balloon_page_enqueue(vb_dev_info);
+
+               if (!page) {
+                       dev_info_ratelimited(&vb->vdev->dev,
+                                            "Out of puff! Can't get %u 
pages\n",
+                                            VIRTIO_BALLOON_PAGES_PER_PAGE);
+                       /* Sleep for at least 1/5 of a second before retry. */
+                       msleep(200);
+                       break;
+               }
+               set_page_bitmap(vb, page);
+               vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
+               if (!virtio_has_feature(vb->vdev,
+                                       VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
+                       adjust_managed_page_count(page, -1);
+       }
+
+       num_allocated_pages = vb->num_pfns;
+       /* Did we get any? */
+       if (vb->num_pfns != 0)
+               tell_host(vb, vb->inflate_vq);
+       mutex_unlock(&vb->balloon_lock);
+
+       return num_allocated_pages;
+}
+
+static long fill_balloon(struct virtio_balloon *vb, size_t num)
+{
+       long num_allocated_pages;
+
+       if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_PAGE_BITMAP))
+               num_allocated_pages = fill_balloon_bitmap(vb, num);
+       else
+               num_allocated_pages = fill_balloon_pfns(vb, num);
+
+       return num_allocated_pages;
+}
+
+static void release_pages_balloon_bitmap(struct virtio_balloon *vb)
+{
+       unsigned long pfn, offset, size;
+       struct page *page;
+
+       size = min(vb->bmap_len * BITS_PER_BYTE, vb->end_pfn);
+       for (offset = vb->start_pfn; offset < size;
+                offset = pfn + VIRTIO_BALLOON_PAGES_PER_PAGE) {
+               pfn = find_next_bit(vb->page_bitmap, size, offset);
+               if (pfn < size) {
+                       page = balloon_pfn_to_page(pfn);
+                       if (!virtio_has_feature(vb->vdev,
+                                       VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
+                               adjust_managed_page_count(page, 1);
+                       put_page(page);
+               }
+       }
+}
+
+static unsigned long leak_balloon_bitmap(struct virtio_balloon *vb, size_t num)
+{
+       unsigned long num_freed_pages = num;
+       struct page *page;
+       struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
+
+       if (balloon_page_bitmap_init(vb) < 0)
+               return num_freed_pages;
+
+       mutex_lock(&vb->balloon_lock);
+       for (vb->num_pfns = 0; vb->num_pfns < num;
+            vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
+               page = balloon_page_dequeue(vb_dev_info);
+               if (!page)
+                       break;
+               set_page_bitmap(vb, page);
+               vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
+       }
+
+       num_freed_pages = vb->num_pfns;
+
+       if (vb->num_pfns != 0)
+               tell_host(vb, vb->deflate_vq);
+       release_pages_balloon_bitmap(vb);
+       mutex_unlock(&vb->balloon_lock);
+
+       return num_freed_pages;
+}
+
+static void release_pages_balloon_pfns(struct virtio_balloon *vb)
 {
        unsigned int i;
 
@@ -188,7 +355,7 @@ static void release_pages_balloon(struct virtio_balloon *vb)
        }
 }
 
-static unsigned leak_balloon(struct virtio_balloon *vb, size_t num)
+static unsigned leak_balloon_pfns(struct virtio_balloon *vb, size_t num)
 {
        unsigned num_freed_pages;
        struct page *page;
@@ -215,11 +382,23 @@ static unsigned leak_balloon(struct virtio_balloon *vb, 
size_t num)
         */
        if (vb->num_pfns != 0)
                tell_host(vb, vb->deflate_vq);
-       release_pages_balloon(vb);
+       release_pages_balloon_pfns(vb);
        mutex_unlock(&vb->balloon_lock);
        return num_freed_pages;
 }
 
+static long leak_balloon(struct virtio_balloon *vb, size_t num)
+{
+       long num_freed_pages;
+
+       if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_PAGE_BITMAP))
+               num_freed_pages = leak_balloon_bitmap(vb, num);
+       else
+               num_freed_pages = leak_balloon_pfns(vb, num);
+
+       return num_freed_pages;
+}
+
 static inline void update_stat(struct virtio_balloon *vb, int idx,
                               u16 tag, u64 val)
 {
@@ -510,6 +689,8 @@ static int virtballoon_probe(struct virtio_device *vdev)
        spin_lock_init(&vb->stop_update_lock);
        vb->stop_update = false;
        vb->num_pages = 0;
+       vb->page_bitmap = NULL;
+       vb->bmap_len = 0;
        mutex_init(&vb->balloon_lock);
        init_waitqueue_head(&vb->acked);
        vb->vdev = vdev;
@@ -567,6 +748,7 @@ static void virtballoon_remove(struct virtio_device *vdev)
        cancel_work_sync(&vb->update_balloon_stats_work);
 
        remove_common(vb);
+       kfree(vb->page_bitmap);
        kfree(vb);
 }
 
@@ -605,6 +787,7 @@ static unsigned int features[] = {
        VIRTIO_BALLOON_F_MUST_TELL_HOST,
        VIRTIO_BALLOON_F_STATS_VQ,
        VIRTIO_BALLOON_F_DEFLATE_ON_OOM,
+       VIRTIO_BALLOON_F_PAGE_BITMAP,
 };
 
 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..f78fa47 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_PAGE_BITMAP   3 /* Send page info with bitmap */
 
 /* Size of a PFN in the balloon interface. */
 #define VIRTIO_BALLOON_PFN_SHIFT 12
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c1069ef..74b2fc5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2139,6 +2139,12 @@ void drain_all_pages(struct zone *zone)
                                                                zone, 1);
 }
 
+unsigned long get_max_pfn(void)
+{
+       return max_pfn;
+}
+EXPORT_SYMBOL(get_max_pfn);
+
 #ifdef CONFIG_HIBERNATION
 
 void mark_free_pages(struct zone *zone)
-- 
1.9.1