Re: [Qemu-devel] [PATCH kernel v8 2/4] virtio-balloon: VIRTIO_BALLOON_F_CHUNK_TRANSFER

[Wang, Wei W]

On Wednesday, April 5, 2017 11:54 AM, Michael S. Tsirkin wrote:
> On Wed, Apr 05, 2017 at 03:31:36AM +0000, Wang, Wei W wrote:
> > On Thursday, March 16, 2017 3:09 PM Wei Wang wrote:
> > > The implementation of the current virtio-balloon is not very
> > > efficient, because the ballooned 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
> > > chunks. A chunk consists of guest physically continuous pages, and
> > > it is offered to the host via a base PFN (i.e. the start PFN of
> > > those physically continuous pages) and the size (i.e. the total number of 
> > > the
> pages). A chunk is formated as below:
> > >
> > > --------------------------------------------------------
> > > |                 Base (52 bit)        | Rsvd (12 bit) |
> > > --------------------------------------------------------
> > > --------------------------------------------------------
> > > |                 Size (52 bit)        | Rsvd (12 bit) |
> > > --------------------------------------------------------
> > >
> > > By doing so, step 4) can also be optimized by doing address
> > > translation and
> > > madvise() in chunks rather than page by page.
> > >
> > > This optimization requires the negotiation of a new feature bit,
> > > VIRTIO_BALLOON_F_CHUNK_TRANSFER.
> > >
> > > With this new feature, the above ballooning process takes ~590ms
> > > resulting in an improvement of ~85%.
> > >
> > > TODO: optimize stage 1) by allocating/freeing a chunk of pages
> > > instead of a single page each time.
> > >
> > > Signed-off-by: Liang Li <address@hidden>
> > > Signed-off-by: Wei Wang <address@hidden>
> > > Suggested-by: Michael S. Tsirkin <address@hidden>
> > > ---
> > >  drivers/virtio/virtio_balloon.c     | 371
> +++++++++++++++++++++++++++++++++-
> > > --
> > >  include/uapi/linux/virtio_balloon.h |   9 +
> > >  2 files changed, 353 insertions(+), 27 deletions(-)
> > >
> > > diff --git a/drivers/virtio/virtio_balloon.c
> > > b/drivers/virtio/virtio_balloon.c index
> > > f59cb4f..3f4a161 100644
> > > --- a/drivers/virtio/virtio_balloon.c
> > > +++ b/drivers/virtio/virtio_balloon.c
> > > @@ -42,6 +42,10 @@
> > >  #define OOM_VBALLOON_DEFAULT_PAGES 256  #define
> > > VIRTBALLOON_OOM_NOTIFY_PRIORITY 80
> > >
> > > +#define PAGE_BMAP_SIZE   (8 * PAGE_SIZE)
> > > +#define PFNS_PER_PAGE_BMAP       (PAGE_BMAP_SIZE * BITS_PER_BYTE)
> > > +#define PAGE_BMAP_COUNT_MAX      32
> > > +
> > >  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"); @@ -50,6
> +54,14
> > > @@ MODULE_PARM_DESC(oom_pages, "pages to free on OOM");  static
> > > struct vfsmount *balloon_mnt;  #endif
> > >
> > > +#define BALLOON_CHUNK_BASE_SHIFT 12 #define
> > > +BALLOON_CHUNK_SIZE_SHIFT 12 struct balloon_page_chunk {
> > > + __le64 base;
> > > + __le64 size;
> > > +};
> > > +
> > > +typedef __le64 resp_data_t;
> > >  struct virtio_balloon {
> > >   struct virtio_device *vdev;
> > >   struct virtqueue *inflate_vq, *deflate_vq, *stats_vq; @@ -67,6
> > > +79,31 @@ struct virtio_balloon {
> > >
> > >   /* Number of balloon pages we've told the Host we're not using. */
> > >   unsigned int num_pages;
> > > + /* Pointer to the response header. */
> > > + struct virtio_balloon_resp_hdr *resp_hdr;
> > > + /* Pointer to the start address of response data. */
> > > + resp_data_t *resp_data;
> >
> > I think the implementation has an issue here - both the balloon pages and 
> > the
> unused pages use the same buffer ("resp_data" above) to store chunks. It would
> cause a race in this case: live migration starts while ballooning is also in 
> progress.
> I plan to use separate buffers for CHUNKS_OF_BALLOON_PAGES and
> CHUNKS_OF_UNUSED_PAGES. Please let me know if you have a different
> suggestion. Thanks.
> >
> > Best,
> > Wei
> 
> Is only one resp data ever in flight for each kind?
> If not you want as many buffers as vq allows.
> 

No, all the kinds were using only one resp_data. I will make it one resp_data 
for each kind.

Best,
Wei