Re: [Qemu-devel] [PATCH v15 3/5] virtio-balloon: VIRTIO_BALLOON_F_SG

[Michael S. Tsirkin]

On Tue, Aug 29, 2017 at 11:09:18AM +0800, Wei Wang wrote:
> On 08/29/2017 02:03 AM, Michael S. Tsirkin wrote:
> > On Mon, Aug 28, 2017 at 06:08:31PM +0800, Wei Wang wrote:
> > > Add a new feature, VIRTIO_BALLOON_F_SG, which enables the transfer
> > > of balloon (i.e. inflated/deflated) pages using scatter-gather lists
> > > to the host.
> > > 
> > > The implementation of the previous virtio-balloon is not very
> > > efficient, because the balloon 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
> > > sgs. An sg describes a chunk of guest physically continuous pages.
> > > With this mechanism, step 4) can also be optimized by doing address
> > > translation and madvise() in chunks rather than page by page.
> > > 
> > > With this new feature, the above ballooning process takes ~597ms
> > > resulting in an improvement of ~86%.
> > > 
> > > TODO: optimize stage 1) by allocating/freeing a chunk of pages
> > > instead of a single page each time.
> > > 
> > > Signed-off-by: Wei Wang <address@hidden>
> > > Signed-off-by: Liang Li <address@hidden>
> > > Suggested-by: Michael S. Tsirkin <address@hidden>
> > > ---
> > >   drivers/virtio/virtio_balloon.c     | 171 
> > > ++++++++++++++++++++++++++++++++----
> > >   include/uapi/linux/virtio_balloon.h |   1 +
> > >   2 files changed, 155 insertions(+), 17 deletions(-)
> > > 
> > > diff --git a/drivers/virtio/virtio_balloon.c 
> > > b/drivers/virtio/virtio_balloon.c
> > > index f0b3a0b..8ecc1d4 100644
> > > --- a/drivers/virtio/virtio_balloon.c
> > > +++ b/drivers/virtio/virtio_balloon.c
> > > @@ -32,6 +32,8 @@
> > >   #include <linux/mm.h>
> > >   #include <linux/mount.h>
> > >   #include <linux/magic.h>
> > > +#include <linux/xbitmap.h>
> > > +#include <asm/page.h>
> > >   /*
> > >    * Balloon device works in 4K page units.  So each page is pointed to by
> > > @@ -79,6 +81,9 @@ struct virtio_balloon {
> > >           /* Synchronize access/update to this struct virtio_balloon 
> > > elements */
> > >           struct mutex balloon_lock;
> > > + /* The xbitmap used to record balloon pages */
> > > + struct xb page_xb;
> > > +
> > >           /* The array of pfns we tell the Host about. */
> > >           unsigned int num_pfns;
> > >           __virtio32 pfns[VIRTIO_BALLOON_ARRAY_PFNS_MAX];
> > > @@ -141,13 +146,111 @@ static void set_page_pfns(struct virtio_balloon 
> > > *vb,
> > >                                             page_to_balloon_pfn(page) + 
> > > i);
> > >   }
> > > +static int add_one_sg(struct virtqueue *vq, void *addr, uint32_t size)
> > > +{
> > > + struct scatterlist sg;
> > > +
> > > + sg_init_one(&sg, addr, size);
> > > + return virtqueue_add_inbuf(vq, &sg, 1, vq, GFP_KERNEL);
> > > +}
> > > +
> > > +static void send_balloon_page_sg(struct virtio_balloon *vb,
> > > +                          struct virtqueue *vq,
> > > +                          void *addr,
> > > +                          uint32_t size,
> > > +                          bool batch)
> > > +{
> > > + unsigned int len;
> > > + int err;
> > > +
> > > + err = add_one_sg(vq, addr, size);
> > > + /* Sanity check: this can't really happen */
> > > + WARN_ON(err);
> > It might be cleaner to detect that add failed due to
> > ring full and kick then. Just an idea, up to you
> > whether to do it.
> > 
> > > +
> > > + /* If batching is in use, we batch the sgs till the vq is full. */
> > > + if (!batch || !vq->num_free) {
> > > +         virtqueue_kick(vq);
> > > +         wait_event(vb->acked, virtqueue_get_buf(vq, &len));
> > > +         /* Release all the entries if there are */
> > Meaning
> >     Account for all used entries if any
> > ?
> > 
> > > +         while (virtqueue_get_buf(vq, &len))
> > > +                 ;
> > 
> > Above code is reused below. Add a function?
> > 
> > > + }
> > > +}
> > > +
> > > +/*
> > > + * Send balloon pages in sgs to host. The balloon pages are recorded in 
> > > the
> > > + * page xbitmap. Each bit in the bitmap corresponds to a page of 
> > > PAGE_SIZE.
> > > + * The page xbitmap is searched for continuous "1" bits, which correspond
> > > + * to continuous pages, to chunk into sgs.
> > > + *
> > > + * @page_xb_start and @page_xb_end form the range of bits in the xbitmap 
> > > that
> > > + * need to be searched.
> > > + */
> > > +static void tell_host_sgs(struct virtio_balloon *vb,
> > > +                   struct virtqueue *vq,
> > > +                   unsigned long page_xb_start,
> > > +                   unsigned long page_xb_end)
> > > +{
> > > + unsigned long sg_pfn_start, sg_pfn_end;
> > > + void *sg_addr;
> > > + uint32_t sg_len, sg_max_len = round_down(UINT_MAX, PAGE_SIZE);
> > > +
> > > + sg_pfn_start = page_xb_start;
> > > + while (sg_pfn_start < page_xb_end) {
> > > +         sg_pfn_start = xb_find_next_bit(&vb->page_xb, sg_pfn_start,
> > > +                                         page_xb_end, 1);
> > > +         if (sg_pfn_start == page_xb_end + 1)
> > > +                 break;
> > > +         sg_pfn_end = xb_find_next_bit(&vb->page_xb, sg_pfn_start + 1,
> > > +                                       page_xb_end, 0);
> > > +         sg_addr = (void *)pfn_to_kaddr(sg_pfn_start);
> > > +         sg_len = (sg_pfn_end - sg_pfn_start) << PAGE_SHIFT;
> > > +         while (sg_len > sg_max_len) {
> > > +                 send_balloon_page_sg(vb, vq, sg_addr, sg_max_len, 1);
> > Last argument should be true, not 1.
> > 
> > > +                 sg_addr += sg_max_len;
> > > +                 sg_len -= sg_max_len;
> > > +         }
> > > +         send_balloon_page_sg(vb, vq, sg_addr, sg_len, 1);
> > > +         xb_zero(&vb->page_xb, sg_pfn_start, sg_pfn_end);
> > > +         sg_pfn_start = sg_pfn_end + 1;
> > > + }
> > > +
> > > + /*
> > > +  * The last few sgs may not reach the batch size, but need a kick to
> > > +  * notify the device to handle them.
> > > +  */
> > > + if (vq->num_free != virtqueue_get_vring_size(vq)) {
> > > +         virtqueue_kick(vq);
> > > +         wait_event(vb->acked, virtqueue_get_buf(vq, &sg_len));
> > > +         while (virtqueue_get_buf(vq, &sg_len))
> > > +                 ;
> > Some entries can get used after a pause. Looks like they will leak then?
> > One fix would be to convert above if to a while loop.
> > I don't know whether to do it like this in send_balloon_page_sg too.
> > 
> 
> Thanks for the above comments. I've re-written this part of code.
> Please have a check below if there is anything more we could improve:
> 
> static void kick_and_wait(struct virtqueue *vq, wait_queue_head_t wq_head)
> {
>         unsigned int len;
> 
>         virtqueue_kick(vq);
>         wait_event(wq_head, virtqueue_get_buf(vq, &len));
>         /* Detach all the used buffers from the vq */
>         while (virtqueue_get_buf(vq, &len))
>                 ;

I would move this last part to before add_buf. Increases chances
it succeeds even in case of a bug.

> }
> 
> static int add_one_sg(struct virtqueue *vq, void *addr, uint32_t size)
> {
>         struct scatterlist sg;
>         int ret;
> 
>         sg_init_one(&sg, addr, size);
>         ret = virtqueue_add_inbuf(vq, &sg, 1, vq, GFP_KERNEL);
>         if (unlikely(ret == -ENOSPC))
>                 dev_warn(&vq->vdev->dev, "%s: failed due to ring full\n",
>                                  __func__);

So if this ever triggers then kick and wait might fail, right?
I think you should not special-case this one then.

> 
>         return ret;
> }
> 
> static void send_balloon_page_sg(struct virtio_balloon *vb,
>                                                       struct virtqueue *vq,
>                                                       void *addr,
>                                                       uint32_t size,
>                                                       bool batch)
> {
>         int err;
> 
>         do {
>                 err = add_one_sg(vq, addr, size);
>                 if (err == -ENOSPC || !batch || !vq->num_free)
>                         kick_and_wait(vq, vb->acked);
>         } while (err == -ENOSPC);
> }
> 
> 
> Best,
> Wei

I think this fixes the bug, yes. I would skip trying to handle ENOSPC,
it's more a less a bug if it triggers. Just bail out on any error.

-- 
MST