Re: [virtio-dev] [RFC for qemu v4 2/2] virtio_balloon: Add dcvq to deflate continuous pages

[teawater <teawaterz@linux.alibaba.com>]

> 2020å7æ16æ 14:39ïMichael S. Tsirkin <mst@redhat.com> åéï
> 
> On Thu, Jul 16, 2020 at 10:41:55AM +0800, Hui Zhu wrote:
>> This commit adds a vq dcvq to deflate continuous pages.
>> When VIRTIO_BALLOON_F_CONT_PAGES is set, try to get continuous pages
>> from icvq and use madvise MADV_WILLNEED with the pages.
>> 
>> Signed-off-by: Hui Zhu <teawaterz@linux.alibaba.com>
> 
> This is arguably something to benchmark. Does guest benefit
> from MADV_WILLNEED or loose performance?

MADV_WILLNEED will call madvise_willneed in the host kernel.
madvise_willneed will schedule all required I/O operations (swap in or vfs_fadvise POSIX_FADV_WILLNEED) of the address.

But the pages of the balloon are released by MADV_DONTNEED.
So I think MADV_WILLNEED will not affect the performance of the guest in the most of situations.

Best,
Hui

> 
>> ---
>> hw/virtio/virtio-balloon.c         | 14 +++++++++-----
>> include/hw/virtio/virtio-balloon.h |  2 +-
>> 2 files changed, 10 insertions(+), 6 deletions(-)
>> 
>> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
>> index d36a5c8..165adf7 100644
>> --- a/hw/virtio/virtio-balloon.c
>> +++ b/hw/virtio/virtio-balloon.c
>> @@ -138,7 +138,8 @@ static void balloon_inflate_page(VirtIOBalloon *balloon,
>> }
>> 
>> static void balloon_deflate_page(VirtIOBalloon *balloon,
>> -                                 MemoryRegion *mr, hwaddr mr_offset)
>> +                                 MemoryRegion *mr, hwaddr mr_offset,
>> +                                 size_t size)
>> {
>>     void *addr = memory_region_get_ram_ptr(mr) + mr_offset;
>>     ram_addr_t rb_offset;
>> @@ -153,10 +154,11 @@ static void balloon_deflate_page(VirtIOBalloon *balloon,
>>     rb_page_size = qemu_ram_pagesize(rb);
>> 
>>     host_addr = (void *)((uintptr_t)addr & ~(rb_page_size - 1));
>> +    size &= ~(rb_page_size - 1);
>> 
>>     /* When a page is deflated, we hint the whole host page it lives
>>      * on, since we can't do anything smaller */
>> -    ret = qemu_madvise(host_addr, rb_page_size, QEMU_MADV_WILLNEED);
>> +    ret = qemu_madvise(host_addr, size, QEMU_MADV_WILLNEED);
>>     if (ret != 0) {
>>         warn_report("Couldn't MADV_WILLNEED on balloon deflate: %s",
>>                     strerror(errno));
>> @@ -354,7 +356,7 @@ static void virtio_balloon_handle_output(VirtIODevice *vdev, VirtQueue *vq)
>>             pa = (hwaddr) p << VIRTIO_BALLOON_PFN_SHIFT;
>>             offset += 4;
>> 
>> -            if (vq == s->icvq) {
>> +            if (vq == s->icvq || vq == s->dcvq) {
>>                 uint32_t psize_ptr;
>>                 if (iov_to_buf(elem->out_sg, elem->out_num, offset, &psize_ptr, 4) != 4) {
>>                     break;
>> @@ -383,8 +385,9 @@ static void virtio_balloon_handle_output(VirtIODevice *vdev, VirtQueue *vq)
>>                     balloon_inflate_page(s, section.mr,
>>                                          section.offset_within_region,
>>                                          psize, &pbp);
>> -                } else if (vq == s->dvq) {
>> -                    balloon_deflate_page(s, section.mr, section.offset_within_region);
>> +                } else if (vq == s->dvq || vq == s->dcvq) {
>> +                    balloon_deflate_page(s, section.mr, section.offset_within_region,
>> +                                         psize);
>>                 } else {
>>                     g_assert_not_reached();
>>                 }
>> @@ -838,6 +841,7 @@ static void virtio_balloon_device_realize(DeviceState *dev, Error **errp)
>> 
>>     if (virtio_has_feature(s->host_features, VIRTIO_BALLOON_F_CONT_PAGES)) {
>>         s->icvq = virtio_add_queue(vdev, 128, virtio_balloon_handle_output);
>> +        s->dcvq = virtio_add_queue(vdev, 128, virtio_balloon_handle_output);
>>     }
>> 
>>     reset_stats(s);
>> diff --git a/include/hw/virtio/virtio-balloon.h b/include/hw/virtio/virtio-balloon.h
>> index 6a2514d..848a7fb 100644
>> --- a/include/hw/virtio/virtio-balloon.h
>> +++ b/include/hw/virtio/virtio-balloon.h
>> @@ -42,7 +42,7 @@ enum virtio_balloon_free_page_report_status {
>> 
>> typedef struct VirtIOBalloon {
>>     VirtIODevice parent_obj;
>> -    VirtQueue *ivq, *dvq, *svq, *free_page_vq, *icvq;
>> +    VirtQueue *ivq, *dvq, *svq, *free_page_vq, *icvq, *dcvq;
>>     uint32_t free_page_report_status;
>>     uint32_t num_pages;
>>     uint32_t actual;
>> -- 
>> 2.7.4
> 
> 
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