Re: [RFC 2/2] virtio_balloon: auto-ballooning support
From: Michael S. Tsirkin
Date: Sun May 12 2013 - 10:31:15 EST
On Thu, May 09, 2013 at 10:53:49AM -0400, Luiz Capitulino wrote:
> Automatic ballooning consists of dynamically adjusting the guest's
> balloon according to memory pressure in the host and in the guest.
>
> This commit implements the guest side of automatic balloning, which
> basically consists of registering a shrinker callback with the kernel,
> which will try to deflate the guest's balloon by the amount of pages
> being requested. The shrinker callback is only registered if the host
> supports the VIRTIO_BALLOON_F_AUTO_BALLOON feature bit.
OK so we have a new feature bit, such that:
- if AUTO_BALLOON is set in host, guest can leak a
page from a balloon at any time
questions left unanswered
- what meaning does num_pages have now?
- when will the balloon be re-inflated?
I'd like to see a spec patch addressing these questions.
Would we ever want to mix the two types of ballooning?
If yes possibly when we put a page in the balloon we
might want to give host a hint "this page might be
leaked again soon".
> Automatic inflate is performed by the host.
>
> Here are some numbers. The test-case is to run 35 VMs (1G of RAM each)
> in parallel doing a kernel build. Host has 32GB of RAM and 16GB of swap.
> SWAP IN and SWAP OUT correspond to the number of pages swapped in and
> swapped out, respectively.
>
> Auto-ballooning disabled:
>
> RUN TIME(s) SWAP IN SWAP OUT
>
> 1 634 930980 1588522
> 2 610 627422 1362174
> 3 649 1079847 1616367
> 4 543 953289 1635379
> 5 642 913237 1514000
>
> Auto-ballooning enabled:
>
> RUN TIME(s) SWAP IN SWAP OUT
>
> 1 629 901 12537
> 2 624 981 18506
> 3 626 573 9085
> 4 631 2250 42534
> 5 627 1610 20808
So what exactly happens here?
Much less swap in/out activity, but no gain
in total runtime. Doesn't this show there's
a bottleneck somewhere? Could be a problem in
the implementation?
Also, what happened with the balloon?
Did we end up with balloon completely inflated? deflated?
One question to consider: possibly if we are
going to reuse the page in the balloon soon,
we might want to bypass notify before use for it?
Maybe that will help speed things up.
> Signed-off-by: Luiz Capitulino <lcapitulino@xxxxxxxxxx>
> ---
> drivers/virtio/virtio_balloon.c | 55 +++++++++++++++++++++++++++++++++++++
> include/uapi/linux/virtio_balloon.h | 1 +
> 2 files changed, 56 insertions(+)
>
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 9d5fe2b..f9dcae8 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -71,6 +71,9 @@ struct virtio_balloon
> /* Memory statistics */
> int need_stats_update;
> struct virtio_balloon_stat stats[VIRTIO_BALLOON_S_NR];
> +
> + /* Memory shrinker */
> + struct shrinker shrinker;
> };
>
> static struct virtio_device_id id_table[] = {
> @@ -126,6 +129,7 @@ static void set_page_pfns(u32 pfns[], struct page *page)
> pfns[i] = page_to_balloon_pfn(page) + i;
> }
>
> +/* This function should be called with vb->balloon_mutex held */
> static void fill_balloon(struct virtio_balloon *vb, size_t num)
> {
> struct balloon_dev_info *vb_dev_info = vb->vb_dev_info;
> @@ -166,6 +170,7 @@ static void release_pages_by_pfn(const u32 pfns[], unsigned int num)
> }
> }
>
> +/* This function should be called with vb->balloon_mutex held */
> static void leak_balloon(struct virtio_balloon *vb, size_t num)
> {
> struct page *page;
> @@ -285,6 +290,45 @@ static void update_balloon_size(struct virtio_balloon *vb)
> &actual, sizeof(actual));
> }
>
> +static unsigned long balloon_get_nr_pages(const struct virtio_balloon *vb)
> +{
> + return vb->num_pages / VIRTIO_BALLOON_PAGES_PER_PAGE;
> +}
> +
> +static int balloon_shrinker(struct shrinker *shrinker,struct shrink_control *sc)
> +{
> + unsigned int nr_pages, new_target;
> + struct virtio_balloon *vb;
> +
> + vb = container_of(shrinker, struct virtio_balloon, shrinker);
> + if (!mutex_trylock(&vb->balloon_lock)) {
> + return -1;
> + }
> +
> + nr_pages = balloon_get_nr_pages(vb);
> + if (!sc->nr_to_scan || !nr_pages) {
> + goto out;
> + }
> +
> + /*
> + * If the current balloon size is greater than the number of
> + * pages being reclaimed by the kernel, deflate only the needed
> + * amount. Otherwise deflate everything we have.
> + */
> + new_target = 0;
> + if (nr_pages > sc->nr_to_scan) {
> + new_target = nr_pages - sc->nr_to_scan;
> + }
> +
> + leak_balloon(vb, new_target);
> + update_balloon_size(vb);
> + nr_pages = balloon_get_nr_pages(vb);
> +
> +out:
> + mutex_unlock(&vb->balloon_lock);
> + return nr_pages;
> +}
> +
> static int balloon(void *_vballoon)
> {
> struct virtio_balloon *vb = _vballoon;
> @@ -471,6 +515,13 @@ static int virtballoon_probe(struct virtio_device *vdev)
> goto out_del_vqs;
> }
>
> + memset(&vb->shrinker, 0, sizeof(vb->shrinker));
> + if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_AUTO_BALLOON)) {
> + vb->shrinker.shrink = balloon_shrinker;
> + vb->shrinker.seeks = DEFAULT_SEEKS;
> + register_shrinker(&vb->shrinker);
> + }
> +
> return 0;
>
> out_del_vqs:
> @@ -487,6 +538,9 @@ out:
>
> static void remove_common(struct virtio_balloon *vb)
> {
> + if (vb->shrinker.shrink)
> + unregister_shrinker(&vb->shrinker);
> +
> /* There might be pages left in the balloon: free them. */
> mutex_lock(&vb->balloon_lock);
> while (vb->num_pages)
> @@ -543,6 +597,7 @@ static int virtballoon_restore(struct virtio_device *vdev)
> static unsigned int features[] = {
> VIRTIO_BALLOON_F_MUST_TELL_HOST,
> VIRTIO_BALLOON_F_STATS_VQ,
> + VIRTIO_BALLOON_F_AUTO_BALLOON,
> };
>
> static struct virtio_driver virtio_balloon_driver = {
> diff --git a/include/uapi/linux/virtio_balloon.h b/include/uapi/linux/virtio_balloon.h
> index 5e26f61..bd378a4 100644
> --- a/include/uapi/linux/virtio_balloon.h
> +++ b/include/uapi/linux/virtio_balloon.h
> @@ -31,6 +31,7 @@
> /* The feature bitmap for virtio balloon */
> #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_AUTO_BALLOON 2 /* Automatic ballooning */
>
> /* Size of a PFN in the balloon interface. */
> #define VIRTIO_BALLOON_PFN_SHIFT 12
> --
> 1.8.1.4
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