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

[Wei Wang <wei.w.wang@intel.com>]

On 09/08/2017 11:36 AM, Michael S. Tsirkin wrote:
> 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@intel.com>
>>>> Signed-off-by: Liang Li <liang.z.li@intel.com>
>>>> Suggested-by: Michael S. Tsirkin <mst@redhat.com>
>>>> ---
>>>>    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.

OK, I will remove the check above, and take other suggestions as well. 
Thanks.

Best,
Wei