From: Jack Thomson <[email protected]> This RFC patch adds a new virtio feature for the virtio-balloon driver during free page hinting, which will wait on device ack before committing the range to the free_page_list. The reason for the change is it allows the device to modify this range without it being reclaimed from the free_page_list before the ack is sent. As expected, testing shows this adds overhead to the hinting run duration, increasing it by ~30% with our Firecracker setup. Currently free page hinting is used mainly for live migration, but this would open it up for a new use-case.
We would like to leverage this with MADV_DONTNEED to reduce RSS of a guest. We'd like to use hinting because of the flexibility of control it brings compared to reporting, allowing memory to be reclaimed in deterministic periods. The traditional balloon device was tested to be much slower when compared to hinting for these workloads. Currently, without this synchronization, hinted pages may be reclaimed from the free list before the device finishes processing them, making hinting unsuitable for this use-case. Signed-off-by: Jack Thomson <[email protected]> --- drivers/virtio/virtio_balloon.c | 21 ++++++++++++++++++--- include/uapi/linux/virtio_balloon.h | 1 + 2 files changed, 19 insertions(+), 3 deletions(-) diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c index 74fe59f5a78c..82b560422279 100644 --- a/drivers/virtio/virtio_balloon.c +++ b/drivers/virtio/virtio_balloon.c @@ -596,8 +596,11 @@ static int init_vqs(struct virtio_balloon *vb) vqs_info[VIRTIO_BALLOON_VQ_STATS].callback = stats_request; } - if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) + if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) { vqs_info[VIRTIO_BALLOON_VQ_FREE_PAGE].name = "free_page_vq"; + if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_HINT_WAIT_ON_ACK)) + vqs_info[VIRTIO_BALLOON_VQ_FREE_PAGE].callback = balloon_ack; + } if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING)) { vqs_info[VIRTIO_BALLOON_VQ_REPORTING].name = "reporting_vq"; @@ -669,8 +672,11 @@ static int send_cmd_id_start(struct virtio_balloon *vb) virtio_balloon_cmd_id_received(vb)); sg_init_one(&sg, &vb->cmd_id_active, sizeof(vb->cmd_id_active)); err = virtqueue_add_outbuf(vq, &sg, 1, &vb->cmd_id_active, GFP_KERNEL); - if (!err) + if (!err) { virtqueue_kick(vq); + if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_HINT_WAIT_ON_ACK)) + wait_event(vb->acked, virtqueue_get_buf(vq, &unused)); + } return err; } @@ -686,8 +692,11 @@ static int send_cmd_id_stop(struct virtio_balloon *vb) sg_init_one(&sg, &vb->cmd_id_stop, sizeof(vb->cmd_id_stop)); err = virtqueue_add_outbuf(vq, &sg, 1, &vb->cmd_id_stop, GFP_KERNEL); - if (!err) + if (!err) { virtqueue_kick(vq); + if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_HINT_WAIT_ON_ACK)) + wait_event(vb->acked, virtqueue_get_buf(vq, &unused)); + } return err; } @@ -722,6 +731,8 @@ static int get_free_page_and_send(struct virtio_balloon *vb) return err; } virtqueue_kick(vq); + if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_HINT_WAIT_ON_ACK)) + wait_event(vb->acked, virtqueue_get_buf(vq, &unused)); spin_lock_irq(&vb->free_page_list_lock); balloon_page_push(&vb->free_page_list, page); vb->num_free_page_blocks++; @@ -1186,6 +1197,9 @@ static int virtballoon_validate(struct virtio_device *vdev) else if (!virtio_has_feature(vdev, VIRTIO_BALLOON_F_PAGE_POISON)) __virtio_clear_bit(vdev, VIRTIO_BALLOON_F_REPORTING); + if (!virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) + __virtio_clear_bit(vdev, VIRTIO_BALLOON_F_HINT_WAIT_ON_ACK); + __virtio_clear_bit(vdev, VIRTIO_F_ACCESS_PLATFORM); return 0; } @@ -1197,6 +1211,7 @@ static unsigned int features[] = { VIRTIO_BALLOON_F_FREE_PAGE_HINT, VIRTIO_BALLOON_F_PAGE_POISON, VIRTIO_BALLOON_F_REPORTING, + VIRTIO_BALLOON_F_HINT_WAIT_ON_ACK, }; static struct virtio_driver virtio_balloon_driver = { diff --git a/include/uapi/linux/virtio_balloon.h b/include/uapi/linux/virtio_balloon.h index ee35a372805d..86698ab06261 100644 --- a/include/uapi/linux/virtio_balloon.h +++ b/include/uapi/linux/virtio_balloon.h @@ -37,6 +37,7 @@ #define VIRTIO_BALLOON_F_FREE_PAGE_HINT 3 /* VQ to report free pages */ #define VIRTIO_BALLOON_F_PAGE_POISON 4 /* Guest is using page poisoning */ #define VIRTIO_BALLOON_F_REPORTING 5 /* Page reporting virtqueue */ +#define VIRTIO_BALLOON_F_HINT_WAIT_ON_ACK 6 /* Page hinting waits on device ack */ /* Size of a PFN in the balloon interface. */ #define VIRTIO_BALLOON_PFN_SHIFT 12 base-commit: 24d479d26b25bce5faea3ddd9fa8f3a6c3129ea7 -- 2.43.0
