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

[Wang, Wei W]

On Wednesday, April 5, 2017 12:31 PM, Wei Wang wrote:
> 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.
> 

Just in case it wasn't well explained - it is one resp data in flight for each 
kind, but the two kinds share the one resp data under the balloon_lock.  I'm 
thinking would it be worthwhile to have them use separate resp data, so that 
when live migration begins, it doesn't need to wait to get unused pages (in the 
case when "inflating" is using resp data and not sure how long would it take to 
finish). The unused pages are useful for the first round of ram transfer, so I 
think it is important to get them as soon as possible.

Best,
Wei