[Devel] [PATCH RHEL9 COMMIT] drivers/vhost: add ioctl to increase the number of workers

[Konstantin Khorenko]

The commit is pushed to "branch-rh9-5.14.0-70.22.1.vz9.17.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh9-5.14.0-70.22.1.vz9.17.3
------>
commit 83860720697fc673ea32138b92835f40ebdbea60
Author: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
Date:   Thu Sep 8 18:32:53 2022 +0300

    drivers/vhost: add ioctl to increase the number of workers
    
    Finally add ioctl to allow userspace to create additional workers
    For now only allow to increase the number of workers
    
    https://jira.sw.ru/browse/PSBM-139414
    Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
    
    ======
    Patchset description:
    vhost-blk: in-kernel accelerator for virtio-blk guests
    
    Although QEMU virtio-blk is quite fast, there is still some room for
    improvements. Disk latency can be reduced if we handle virito-blk
    requests in host kernel so we avoid a lot of syscalls and context
    switches.
    The idea is quite simple - QEMU gives us block device and we translate
    any incoming virtio requests into bio and push them into bdev.
    The biggest disadvantage of this vhost-blk flavor is raw format.
    
    Luckily Kirill Thai proposed device mapper driver for QCOW2 format to
    attach files as block devices:
    https://www.spinics.net/lists/kernel/msg4292965.html
    
    Also by using kernel modules we can bypass iothread limitation and
    finaly scale block requests with cpus for high-performance devices.
    
    There have already been several attempts to write vhost-blk:
    
    Asias'   version:       https://lkml.org/lkml/2012/12/1/174
    Badari's version:       https://lwn.net/Articles/379864/
    Vitaly's version:       https://lwn.net/Articles/770965/
    
    The main difference between them is API to access backend file. The
    fastest one is Asias's version with bio flavor. It is also the most
    reviewed and have the most features. So vhost_blk module is partially
    based on it. Multiple virtqueue support was addded, some places
    reworked. Added support for several vhost workers.
    
    test setup and results:
      fio --direct=1 --rw=randread  --bs=4k  --ioengine=libaio --iodepth=128
    QEMU drive options: cache=none
    filesystem: xfs
    
    SSD:
                   | randread, IOPS  | randwrite, IOPS |
    Host           |      95.8k      |      85.3k      |
    QEMU virtio    |      57.5k      |      79.4k      |
    QEMU vhost-blk |      95.6k      |      84.3k      |
    
    RAMDISK (vq == vcpu):
                     | randread, IOPS | randwrite, IOPS |
    virtio, 1vcpu    |      123k      |      129k       |
    virtio, 2vcpu    |      253k (??) |      250k (??)  |
    virtio, 4vcpu    |      158k      |      154k       |
    vhost-blk, 1vcpu |      110k      |      113k       |
    vhost-blk, 2vcpu |      247k      |      252k       |
    vhost-blk, 8vcpu |      497k      |      469k       | *single kernel thread
    vhost-blk, 8vcpu |      730k      |      701k       | *two kernel threads
    
    v2:
    
    patch 1/10
     - removed unused VHOST_BLK_VQ
     - reworked bio handling a bit: now add all pages from signle iov into
    single bio istead of allocating one bio per page
     - changed how to calculate sector incrementation
     - check move_iovec() in vhost_blk_req_handle()
     - remove snprintf check and better check ret from copy_to_iter for
    VIRTIO_BLK_ID_BYTES requests
     - discard vq request if vhost_blk_req_handle() returned negative code
     - forbid to change nonzero backend in vhost_blk_set_backend(). First of
    all, QEMU sets backend only once. Also if we want to change backend when
    we already running requests we need to be much more careful in
    vhost_blk_handle_guest_kick() as it is not taking any references. If
    userspace want to change backend that bad it can always reset device.
     - removed EXPERIMENTAL from Kconfig
    
    patch 3/10
     - don't bother with checking dev->workers[0].worker since dev->nworkers
    will always contain 0 in this case
    
    patch 6/10
     - Make code do what docs suggest. Previously ioctl-supplied new number
    of workers were treated like an amount that should be added. Use new
    number as a ceiling instead and add workers up to that number.
    
    https://jira.sw.ru/browse/PSBM-139414
    
    Andrey Zhadchenko (10):
      drivers/vhost: vhost-blk accelerator for virtio-blk guests
      drivers/vhost: use array to store workers
      drivers/vhost: adjust vhost to flush all workers
      drivers/vhost: rework attaching cgroups to be worker aware
      drivers/vhost: rework worker creation
      drivers/vhost: add ioctl to increase the number of workers
      drivers/vhost: assign workers to virtqueues
      drivers/vhost: add API to queue work at virtqueue worker
      drivers/vhost: allow polls to be bound to workers via vqs
      drivers/vhost: queue vhost_blk works at vq workers
    
    Feature: vhost-blk: in-kernel accelerator for virtio-blk guests
---
 drivers/vhost/vhost.c      | 32 +++++++++++++++++++++++++++++++-
 include/uapi/linux/vhost.h |  9 +++++++++
 2 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 0542eab1e815..9066241e8dc6 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -656,6 +656,25 @@ static int vhost_add_worker(struct vhost_dev *dev)
 	return err;
 }
 
+static int vhost_set_workers(struct vhost_dev *dev, int n)
+{
+	int i, ret;
+
+	if (n > dev->nvqs)
+		n = dev->nvqs;
+
+	if (n > VHOST_MAX_WORKERS)
+		n = VHOST_MAX_WORKERS;
+
+	for (i = 0; i < n - dev->nworkers ; i++) {
+		ret = vhost_add_worker(dev);
+		if (ret)
+			break;
+	}
+
+	return ret;
+}
+
 /* Caller should have device mutex */
 long vhost_dev_set_owner(struct vhost_dev *dev)
 {
@@ -1809,7 +1828,7 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
 	struct eventfd_ctx *ctx;
 	u64 p;
 	long r;
-	int i, fd;
+	int i, fd, n;
 
 	/* If you are not the owner, you can become one */
 	if (ioctl == VHOST_SET_OWNER) {
@@ -1866,6 +1885,17 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
 		if (ctx)
 			eventfd_ctx_put(ctx);
 		break;
+	case VHOST_SET_NWORKERS:
+		r = get_user(n, (int __user *)argp);
+		if (r < 0)
+			break;
+		if (n < d->nworkers) {
+			r = -EINVAL;
+			break;
+		}
+
+		r = vhost_set_workers(d, n);
+		break;
 	default:
 		r = -ENOIOCTLCMD;
 		break;
diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
index 13caf114bcde..d6d87f6315f6 100644
--- a/include/uapi/linux/vhost.h
+++ b/include/uapi/linux/vhost.h
@@ -71,6 +71,15 @@
 #define VHOST_SET_VRING_ENDIAN _IOW(VHOST_VIRTIO, 0x13, struct vhost_vring_state)
 #define VHOST_GET_VRING_ENDIAN _IOW(VHOST_VIRTIO, 0x14, struct vhost_vring_state)
 
+/* Set number of vhost workers
+ * Currently nuber of vhost workers can only be increased.
+ * All workers are freed upon reset.
+ * If the value is too big it is silently truncated to the maximum number of
+ * supported vhost workers
+ * Even if the error is returned it is possible that some workers were created
+ */
+#define VHOST_SET_NWORKERS _IOW(VHOST_VIRTIO, 0x1F, int)
+
 /* The following ioctls use eventfd file descriptors to signal and poll
  * for events. */