[PATCH v2 repost 4/7] virtio-balloon: speed up inflate/deflate process

Liang Li
2016-07-27T01:23:33Z

The implementation of the current virtio-balloon is not very
efficient, the time spends on different stages of inflating
the balloon to 7GB of a 8GB idle guest:

a. allocating pages (6.5%)
b. sending PFNs to host (68.3%)
c. address translation (6.1%)
d. madvise (19%)

It takes about 4126ms for the inflating process to complete.
Debugging shows that the bottle neck are the stage b and stage d.

If using a bitmap to send the page info instead of the PFNs, we
can reduce the overhead in stage b quite a lot. Furthermore, we
can do the address translation and call madvise() with a bulk of
RAM pages, instead of the current page per page way, 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. With this new feature, inflating the
balloon to 7GB of a 8GB idle guest only takes 590ms, the
performance improvement is about 85%.

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

Signed-off-by: Liang Li <liang.z.li@intel.com>
Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: Amit Shah <amit.shah@redhat.com>
---
drivers/virtio/virtio_balloon.c | 184 +++++++++++++++++++++++++++++++++++-----
1 file changed, 162 insertions(+), 22 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 8d649a2..2d18ff6 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -41,10 +41,28 @@
#define OOM_VBALLOON_DEFAULT_PAGES 256
#define VIRTBALLOON_OOM_NOTIFY_PRIORITY 80

+/*
+ * VIRTIO_BALLOON_PFNS_LIMIT is used to limit the size of page bitmap
+ * to prevent a very large page bitmap, there are two reasons for this:
+ * 1) to save memory.
+ * 2) allocate a large bitmap may fail.
+ *
+ * The actual limit of pfn is determined by:
+ * pfn_limit = min(max_pfn, VIRTIO_BALLOON_PFNS_LIMIT);
+ *
+ * If system has more pages than VIRTIO_BALLOON_PFNS_LIMIT, we will scan
+ * the page list and send the PFNs with several times. To reduce the
+ * overhead of scanning the page list. VIRTIO_BALLOON_PFNS_LIMIT should
+ * be set with a value which can cover most cases.
+ */
+#define VIRTIO_BALLOON_PFNS_LIMIT ((32 * (1ULL << 30)) >> PAGE_SHIFT) /* 32GB */
+
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 +80,15 @@ struct virtio_balloon {

/* Number of balloon pages we've told the Host we're not using. */
unsigned int num_pages;
+ /* Pointer of the bitmap header. */
+ void *bmap_hdr;
+ /* Bitmap and length used to tell the host the pages */
+ unsigned long *page_bitmap;
+ unsigned long bmap_len;
+ /* Pfn limit */
+ unsigned long pfn_limit;
+ /* Used to record the processed pfn range */
+ unsigned long min_pfn, max_pfn, start_pfn, end_pfn;
/*
* The pages we've told the Host we're not using are enqueued
* at vb_dev_info->pages list.
@@ -105,12 +132,45 @@ static void balloon_ack(struct virtqueue *vq)
wake_up(&vb->acked);
}

+static inline void init_pfn_range(struct virtio_balloon *vb)
+{
+ vb->min_pfn = ULONG_MAX;
+ vb->max_pfn = 0;
+}
+
+static inline void update_pfn_range(struct virtio_balloon *vb,
+ struct page *page)
+{
+ unsigned long balloon_pfn = page_to_balloon_pfn(page);
+
+ if (balloon_pfn < vb->min_pfn)
+ vb->min_pfn = balloon_pfn;
+ if (balloon_pfn > vb->max_pfn)
+ vb->max_pfn = balloon_pfn;
+}
+
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)) {
+ struct balloon_bmap_hdr *hdr = vb->bmap_hdr;
+ unsigned long bmap_len;
+
+ /* cmd and req_id are not used here, set them to 0 */
+ hdr->cmd = cpu_to_virtio16(vb->vdev, 0);
+ hdr->page_shift = cpu_to_virtio16(vb->vdev, PAGE_SHIFT);
+ hdr->reserved = cpu_to_virtio16(vb->vdev, 0);
+ hdr->req_id = cpu_to_virtio64(vb->vdev, 0);
+ hdr->start_pfn = cpu_to_virtio64(vb->vdev, vb->start_pfn);
+ bmap_len = min(vb->bmap_len,
+ (vb->end_pfn - vb->start_pfn) / BITS_PER_BYTE);
+ hdr->bmap_len = cpu_to_virtio64(vb->vdev, bmap_len);
+ sg_init_one(&sg, hdr,
+ sizeof(struct balloon_bmap_hdr) + bmap_len);
+ } else
+ 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);
@@ -118,7 +178,6 @@ static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)

/* When host has read buffer, this completes via balloon_ack */
wait_event(vb->acked, virtqueue_get_buf(vq, &len));
-
}

static void set_page_pfns(struct virtio_balloon *vb,
@@ -133,13 +192,53 @@ static void set_page_pfns(struct virtio_balloon *vb,
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 list_head *pages, struct virtqueue *vq)
+{
+ unsigned long pfn;
+ struct page *page;
+ bool found;
+
+ vb->min_pfn = rounddown(vb->min_pfn, BITS_PER_LONG);
+ vb->max_pfn = roundup(vb->max_pfn, BITS_PER_LONG);
+ for (pfn = vb->min_pfn; pfn < vb->max_pfn;
+ pfn += vb->pfn_limit) {
+ vb->start_pfn = pfn + vb->pfn_limit;
+ vb->end_pfn = pfn;
+ memset(vb->page_bitmap, 0, vb->bmap_len);
+ found = false;
+ list_for_each_entry(page, pages, lru) {
+ unsigned long balloon_pfn = page_to_balloon_pfn(page);
+
+ if (balloon_pfn < pfn ||
+ balloon_pfn >= pfn + vb->pfn_limit)
+ continue;
+ set_bit(balloon_pfn - pfn, vb->page_bitmap);
+ if (balloon_pfn > vb->end_pfn)
+ vb->end_pfn = balloon_pfn;
+ if (balloon_pfn < vb->start_pfn)
+ vb->start_pfn = balloon_pfn;
+ found = true;
+ }
+ if (found) {
+ vb->start_pfn = rounddown(vb->start_pfn, BITS_PER_LONG);
+ vb->end_pfn = roundup(vb->end_pfn, BITS_PER_LONG);
+ tell_host(vb, vq);
+ }
+ }
+}
+
+static unsigned int fill_balloon(struct virtio_balloon *vb, size_t num,
+ bool use_bmap)
{
struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
- unsigned num_allocated_pages;
+ unsigned int num_allocated_pages;

- /* We can only do one array worth at a time. */
- num = min(num, ARRAY_SIZE(vb->pfns));
+ if (use_bmap)
+ init_pfn_range(vb);
+ else
+ /* We can only do one array worth at a time. */
+ num = min(num, ARRAY_SIZE(vb->pfns));

mutex_lock(&vb->balloon_lock);
for (vb->num_pfns = 0; vb->num_pfns < num;
@@ -154,7 +253,10 @@ 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_bmap)
+ update_pfn_range(vb, page);
+ 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))
@@ -163,8 +265,13 @@ 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 != 0) {
+ if (use_bmap)
+ set_page_bitmap(vb, &vb_dev_info->pages,
+ vb->inflate_vq);
+ else
+ tell_host(vb, vb->inflate_vq);
+ }
mutex_unlock(&vb->balloon_lock);

return num_allocated_pages;
@@ -184,15 +291,19 @@ static void release_pages_balloon(struct virtio_balloon *vb,
}
}

-static unsigned leak_balloon(struct virtio_balloon *vb, size_t num)
+static unsigned int leak_balloon(struct virtio_balloon *vb, size_t num,
+ bool use_bmap)
{
- unsigned num_freed_pages;
+ unsigned int num_freed_pages;
struct page *page;
struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
LIST_HEAD(pages);

- /* We can only do one array worth at a time. */
- num = min(num, ARRAY_SIZE(vb->pfns));
+ if (use_bmap)
+ init_pfn_range(vb);
+ else
+ /* We can only do one array worth at a time. */
+ num = min(num, ARRAY_SIZE(vb->pfns));

mutex_lock(&vb->balloon_lock);
for (vb->num_pfns = 0; vb->num_pfns < num;
@@ -200,7 +311,10 @@ 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_bmap)
+ update_pfn_range(vb, page);
+ else
+ set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
list_add(&page->lru, &pages);
vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
}
@@ -211,9 +325,14 @@ 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);
- release_pages_balloon(vb, &pages);
+ if (vb->num_pfns != 0) {
+ if (use_bmap)
+ set_page_bitmap(vb, &pages, vb->deflate_vq);
+ else
+ tell_host(vb, vb->deflate_vq);
+
+ release_pages_balloon(vb, &pages);
+ }
mutex_unlock(&vb->balloon_lock);
return num_freed_pages;
}
@@ -347,13 +466,15 @@ static int virtballoon_oom_notify(struct notifier_block *self,
struct virtio_balloon *vb;
unsigned long *freed;
unsigned num_freed_pages;
+ bool use_bmap;

vb = container_of(self, struct virtio_balloon, nb);
if (!virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
return NOTIFY_OK;

freed = parm;
- num_freed_pages = leak_balloon(vb, oom_pages);
+ use_bmap = virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_PAGE_BITMAP);
+ num_freed_pages = leak_balloon(vb, oom_pages, use_bmap);
update_balloon_size(vb);
*freed += num_freed_pages;

@@ -373,15 +494,17 @@ static void update_balloon_size_func(struct work_struct *work)
{
struct virtio_balloon *vb;
s64 diff;
+ bool use_bmap;

vb = container_of(work, struct virtio_balloon,
update_balloon_size_work);
+ use_bmap = virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_PAGE_BITMAP);
diff = towards_target(vb);

if (diff > 0)
- diff -= fill_balloon(vb, diff);
+ diff -= fill_balloon(vb, diff, use_bmap);
else if (diff < 0)
- diff += leak_balloon(vb, -diff);
+ diff += leak_balloon(vb, -diff, use_bmap);
update_balloon_size(vb);

if (diff)
@@ -489,7 +612,7 @@ static int virtballoon_migratepage(struct balloon_dev_info *vb_dev_info,
static int virtballoon_probe(struct virtio_device *vdev)
{
struct virtio_balloon *vb;
- int err;
+ int err, hdr_len;

if (!vdev->config->get) {
dev_err(&vdev->dev, "%s failure: config access disabled\n",
@@ -508,6 +631,18 @@ static int virtballoon_probe(struct virtio_device *vdev)
spin_lock_init(&vb->stop_update_lock);
vb->stop_update = false;
vb->num_pages = 0;
+ vb->pfn_limit = VIRTIO_BALLOON_PFNS_LIMIT;
+ vb->pfn_limit = min(vb->pfn_limit, get_max_pfn());
+ vb->bmap_len = ALIGN(vb->pfn_limit, BITS_PER_LONG) /
+ BITS_PER_BYTE + 2 * sizeof(unsigned long);
+ hdr_len = sizeof(struct balloon_bmap_hdr);
+ vb->bmap_hdr = kzalloc(hdr_len + vb->bmap_len, GFP_KERNEL);
+
+ /* Clear the feature bit if memory allocation fails */
+ if (!vb->bmap_hdr)
+ __virtio_clear_bit(vdev, VIRTIO_BALLOON_F_PAGE_BITMAP);
+ else
+ vb->page_bitmap = vb->bmap_hdr + hdr_len;
mutex_init(&vb->balloon_lock);
init_waitqueue_head(&vb->acked);
vb->vdev = vdev;
@@ -541,9 +676,12 @@ out:

static void remove_common(struct virtio_balloon *vb)
{
+ bool use_bmap;
+
+ use_bmap = virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_PAGE_BITMAP);
/* There might be pages left in the balloon: free them. */
while (vb->num_pages)
- leak_balloon(vb, vb->num_pages);
+ leak_balloon(vb, vb->num_pages, use_bmap);
update_balloon_size(vb);

/* Now we reset the device so we can clean up the queues. */
@@ -565,6 +703,7 @@ 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);
}

@@ -603,6 +742,7 @@ 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 = {
--
1.9.1

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