Re: [PATCH v15 3/5] virtio-balloon: VIRTIO_BALLOON_F_SG
From: Michael S. Tsirkin
Date: Thu Sep 07 2017 - 23:37:12 EST
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 <wei.w.wang@xxxxxxxxx>
> > > Signed-off-by: Liang Li <liang.z.li@xxxxxxxxx>
> > > Suggested-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > > ---
> > > 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