Date:	Thu, 19 Jul 2012 08:05:42 -0500
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Asias He <asias@...hat.com>, linux-kernel@...r.kernel.org
Cc:	"Michael S. Tsirkin" <mst@...hat.com>, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH RESEND 5/5] vhost-blk: Add vhost-blk support

Asias He <asias@...hat.com> writes:

> vhost-blk is a in kernel virito-blk device accelerator.
>
> This patch is based on Liu Yuan's implementation with various
> improvements and bug fixes. Notably, this patch makes guest notify and
> host completion processing in parallel which gives about 60% performance
> improvement compared to Liu Yuan's implementation.
>
> Performance evaluation:
> -----------------------------
> The comparison is between kvm tool with usersapce implementation and kvm
> tool with vhost-blk.
>
> 1) Fio with libaio ioengine on Fusion IO device
> With bio-based IO path, sequential read/write, random read/write
> IOPS boost         : 8.4%, 15.3%, 10.4%, 14.6%
> Latency improvement: 8.5%, 15.4%, 10.4%, 15.1%
>
> 2) Fio with vsync ioengine on Fusion IO device
> With bio-based IO path, sequential read/write, random read/write
> IOPS boost         : 10.5%, 4.8%, 5.2%, 5.6%
> Latency improvement: 11.4%, 5.0%, 5.2%, 5.8%
>
> Cc: Michael S. Tsirkin <mst@...hat.com>
> Cc: linux-kernel@...r.kernel.org
> Cc: kvm@...r.kernel.org
> Cc: virtualization@...ts.linux-foundation.org
> Signed-off-by: Asias He <asias@...hat.com>
> ---
>  drivers/vhost/Kconfig  |   10 +
>  drivers/vhost/Makefile |    2 +
>  drivers/vhost/blk.c    |  600 ++++++++++++++++++++++++++++++++++++++++++++++++
>  drivers/vhost/vhost.h  |    5 +
>  include/linux/vhost.h  |    3 +
>  5 files changed, 620 insertions(+)
>  create mode 100644 drivers/vhost/blk.c
>
> diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
> index c387067..fa071a8 100644
> --- a/drivers/vhost/Kconfig
> +++ b/drivers/vhost/Kconfig
> @@ -16,4 +16,14 @@ config VHOST_NET
>  
>  	  To compile this driver as a module, choose M here: the module will
>  	  be called vhost_net.
> +config VHOST_BLK
> +	tristate "Host kernel accelerator for virtio blk (EXPERIMENTAL)"
> +	depends on VHOST && BLOCK && AIO && EVENTFD && EXPERIMENTAL
> +	---help---
> +	  This kernel module can be loaded in host kernel to accelerate
> +	  guest block with virtio_blk. Not to be confused with virtio_blk
> +	  module itself which needs to be loaded in guest kernel.
> +
> +	  To compile this driver as a module, choose M here: the module will
> +	  be called vhost_blk.
>  
> diff --git a/drivers/vhost/Makefile b/drivers/vhost/Makefile
> index cd36885..aa461d5 100644
> --- a/drivers/vhost/Makefile
> +++ b/drivers/vhost/Makefile
> @@ -1,4 +1,6 @@
>  obj-$(CONFIG_VHOST)	+= vhost.o
>  obj-$(CONFIG_VHOST_NET) += vhost_net.o
> +obj-$(CONFIG_VHOST_BLK) += vhost_blk.o
>  
>  vhost_net-y		:= net.o
> +vhost_blk-y		:= blk.o
> diff --git a/drivers/vhost/blk.c b/drivers/vhost/blk.c
> new file mode 100644
> index 0000000..6a94894
> --- /dev/null
> +++ b/drivers/vhost/blk.c
> @@ -0,0 +1,600 @@
> +/*
> + * Copyright (C) 2011 Taobao, Inc.
> + * Author: Liu Yuan <tailai.ly@...bao.com>
> + *
> + * Copyright (C) 2012 Red Hat, Inc.
> + * Author: Asias He <asias@...hat.com>
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2.
> + *
> + * virtio-blk server in host kernel.
> + */
> +
> +#include <linux/miscdevice.h>
> +#include <linux/module.h>
> +#include <linux/vhost.h>
> +#include <linux/virtio_blk.h>
> +#include <linux/eventfd.h>
> +#include <linux/mutex.h>
> +#include <linux/file.h>
> +#include <linux/mmu_context.h>
> +#include <linux/anon_inodes.h>
> +#include <linux/kthread.h>
> +#include <linux/blkdev.h>
> +
> +#include "vhost.h"
> +
> +#define BLK_HDR	0
> +
> +enum {
> +	VHOST_BLK_VQ_REQ = 0,
> +	VHOST_BLK_VQ_MAX = 1,
> +};
> +
> +struct vhost_blk_req {
> +	u16 head;
> +	u8 *status;
> +};
> +
> +struct vhost_blk {
> +	struct task_struct *worker_host_kick;
> +	struct task_struct *worker;
> +	struct vhost_blk_req *reqs;
> +	struct vhost_virtqueue vq;
> +	struct eventfd_ctx *ectx;
> +	struct io_event *ioevent;
> +	struct kioctx *ioctx;
> +	struct vhost_dev dev;
> +	struct file *efile;
> +	u64 ioevent_nr;
> +	bool stop;
> +};
> +
> +static inline int vhost_blk_read_events(struct vhost_blk *blk, long nr)
> +{
> +	mm_segment_t old_fs = get_fs();
> +	int ret;
> +
> +	set_fs(KERNEL_DS);
> +	ret = read_events(blk->ioctx, nr, nr, blk->ioevent, NULL);
> +	set_fs(old_fs);
> +
> +	return ret;
> +}
> +
> +static int vhost_blk_setup(struct vhost_blk *blk)
> +{
> +	struct kioctx *ctx;
> +
> +	if (blk->ioctx)
> +		return 0;
> +
> +	blk->ioevent_nr = blk->vq.num;
> +	ctx = ioctx_alloc(blk->ioevent_nr);
> +	if (IS_ERR(ctx)) {
> +		pr_err("Failed to ioctx_alloc");
> +		return PTR_ERR(ctx);
> +	}

Not that it's very likely that ioctx_alloc will fail in practice.
There's a fixed number of events that can be allocated that's currently
0x10000.  If you have a ring queue size of 1024 (which is normal) then
that limits you to 64 vhost-blk devices.

Realistically, I don't think you can only do aio with vhost-blk because
of this (and many other) limitations.  It's necessary to be able to fall
back to a thread pool because AIO cannot be relied upon.

> +	put_ioctx(ctx);
> +	blk->ioctx = ctx;
> +
> +	blk->ioevent = kmalloc(sizeof(struct io_event) * blk->ioevent_nr,
> +			       GFP_KERNEL);
> +	if (!blk->ioevent) {
> +		pr_err("Failed to allocate memory for io_events");
> +		return -ENOMEM;
> +	}
> +
> +	blk->reqs = kmalloc(sizeof(struct vhost_blk_req) * blk->ioevent_nr,
> +			    GFP_KERNEL);
> +	if (!blk->reqs) {
> +		pr_err("Failed to allocate memory for vhost_blk_req");
> +		return -ENOMEM;
> +	}
> +
> +	return 0;
> +}
> +
> +static inline int vhost_blk_set_status(struct vhost_blk *blk, u8 *statusp,
> +				       u8 status)
> +{
> +	if (copy_to_user(statusp, &status, sizeof(status))) {
> +		vq_err(&blk->vq, "Failed to write status\n");
> +		vhost_discard_vq_desc(&blk->vq, 1);
> +		return -EFAULT;
> +	}
> +
> +	return 0;
> +}
> +
> +static void vhost_blk_enable_vq(struct vhost_blk *blk,
> +				struct vhost_virtqueue *vq)
> +{
> +	wake_up_process(blk->worker_host_kick);
> +}
> +
> +static int vhost_blk_io_submit(struct vhost_blk *blk, struct file *file,
> +			       struct vhost_blk_req *req,
> +			       struct iovec *iov, u64 nr_vecs, loff_t offset,
> +			       int opcode)
> +{
> +	struct kioctx *ioctx = blk->ioctx;
> +	mm_segment_t oldfs = get_fs();
> +	struct kiocb_batch batch;
> +	struct blk_plug plug;
> +	struct kiocb *iocb;
> +	int ret;
> +
> +	if (!try_get_ioctx(ioctx)) {
> +		pr_info("Failed to get ioctx");
> +		return -EAGAIN;
> +	}
> +
> +	atomic_long_inc_not_zero(&file->f_count);
> +	eventfd_ctx_get(blk->ectx);
> +
> +	/* TODO: batch to 1 is not good! */
> +	kiocb_batch_init(&batch, 1);
> +	blk_start_plug(&plug);
> +
> +	iocb = aio_get_req(ioctx, &batch);
> +	if (unlikely(!iocb)) {
> +		ret = -EAGAIN;
> +		goto out;
> +	}
> +
> +	iocb->ki_filp	= file;
> +	iocb->ki_pos	= offset;
> +	iocb->ki_buf	= (void *)iov;
> +	iocb->ki_left	= nr_vecs;
> +	iocb->ki_nbytes	= nr_vecs;
> +	iocb->ki_opcode	= opcode;
> +	iocb->ki_obj.user = req;
> +	iocb->ki_eventfd  = blk->ectx;
> +
> +	set_fs(KERNEL_DS);
> +	ret = aio_setup_iocb(iocb, false);
> +	set_fs(oldfs);
> +	if (unlikely(ret))
> +		goto out_put_iocb;
> +
> +	spin_lock_irq(&ioctx->ctx_lock);
> +	if (unlikely(ioctx->dead)) {
> +		spin_unlock_irq(&ioctx->ctx_lock);
> +		ret = -EINVAL;
> +		goto out_put_iocb;
> +	}
> +	aio_run_iocb(iocb);
> +	spin_unlock_irq(&ioctx->ctx_lock);
> +
> +	aio_put_req(iocb);
> +
> +	blk_finish_plug(&plug);
> +	kiocb_batch_free(ioctx, &batch);
> +	put_ioctx(ioctx);
> +
> +	return ret;
> +out_put_iocb:
> +	aio_put_req(iocb); /* Drop extra ref to req */
> +	aio_put_req(iocb); /* Drop I/O ref to req */
> +out:
> +	put_ioctx(ioctx);
> +	return ret;
> +}
> +
> +static void vhost_blk_flush(struct vhost_blk *blk)
> +{
> +	vhost_poll_flush(&blk->vq.poll);
> +}
> +
> +static struct file *vhost_blk_stop_vq(struct vhost_blk *blk,
> +				      struct vhost_virtqueue *vq)
> +{
> +	struct file *file;
> +
> +	mutex_lock(&vq->mutex);
> +	file = rcu_dereference_protected(vq->private_data,
> +			lockdep_is_held(&vq->mutex));
> +	rcu_assign_pointer(vq->private_data, NULL);
> +	mutex_unlock(&vq->mutex);
> +
> +	return file;
> +
> +}
> +
> +static inline void vhost_blk_stop(struct vhost_blk *blk, struct file **file)
> +{
> +
> +	*file = vhost_blk_stop_vq(blk, &blk->vq);
> +}
> +
> +/* Handle guest request */
> +static int vhost_blk_do_req(struct vhost_virtqueue *vq,
> +			    struct virtio_blk_outhdr *hdr,
> +			    u16 head, u16 out, u16 in,
> +			    struct file *file)
> +{
> +	struct vhost_blk *blk = container_of(vq->dev, struct vhost_blk, dev);
> +	struct iovec *iov = &vq->iov[BLK_HDR + 1];
> +	loff_t offset = hdr->sector << 9;
> +	struct vhost_blk_req *req;
> +	u64 nr_vecs;
> +	int ret = 0;
> +	u8 status;
> +
> +	if (hdr->type == VIRTIO_BLK_T_IN || hdr->type == VIRTIO_BLK_T_GET_ID)
> +		nr_vecs = in - 1;
> +	else
> +		nr_vecs = out - 1;
> +
> +	req		= &blk->reqs[head];
> +	req->head	= head;
> +	req->status	= blk->vq.iov[nr_vecs + 1].iov_base;
> +
> +	switch (hdr->type) {
> +	case VIRTIO_BLK_T_OUT:
> +		ret = vhost_blk_io_submit(blk, file, req, iov, nr_vecs, offset,
> +					  IOCB_CMD_PWRITEV);
> +		break;
> +	case VIRTIO_BLK_T_IN:
> +		ret = vhost_blk_io_submit(blk, file, req, iov, nr_vecs, offset,
> +					  IOCB_CMD_PREADV);
> +		break;
> +	case VIRTIO_BLK_T_FLUSH:
> +		ret = vfs_fsync(file, 1);
> +		status = ret < 0 ? VIRTIO_BLK_S_IOERR : VIRTIO_BLK_S_OK;
> +		ret = vhost_blk_set_status(blk, req->status, status);
> +		if (!ret)
> +			vhost_add_used_and_signal(&blk->dev, vq, head, ret);
> +		break;
> +	case VIRTIO_BLK_T_GET_ID:
> +		/* TODO: need a real ID string */
> +		ret = snprintf(vq->iov[BLK_HDR + 1].iov_base,
> +			       VIRTIO_BLK_ID_BYTES, "VHOST-BLK-DISK");
> +		status = ret < 0 ? VIRTIO_BLK_S_IOERR : VIRTIO_BLK_S_OK;
> +		ret = vhost_blk_set_status(blk, req->status, status);
> +		if (!ret)
> +			vhost_add_used_and_signal(&blk->dev, vq, head,
> +						  VIRTIO_BLK_ID_BYTES);
> +		break;
> +	default:
> +		pr_warn("Unsupported request type %d\n", hdr->type);
> +		vhost_discard_vq_desc(vq, 1);
> +		ret = -EFAULT;
> +		break;
> +	}

There doesn't appear to be any error handling in the event that
vhost_blk_io_submit fails.  It would appear that you leak the ring queue
entry since you never push anything onto the used queue.

I think you need to handle io_submit() failing too with EAGAIN.  Relying
on min nr_events == queue_size seems like a dangerous assumption to me
particularly since queue_size tends to be very large and max_nr_events
is a fixed size global pool.

To properly handle EAGAIN, you effectively need to implement flow
control and back off reading the virtqueue until you can submit requests again.

Of course, the million dollar question is why would using AIO in the
kernel be faster than using AIO in userspace?

When using eventfd, there is no heavy weight exit on the notify path.
It should just be the difference between scheduling a kernel thread vs
scheduling a userspace thread.  There's simply no way that that's a 60%
difference in performance.

Regards,

Anthony Liguori
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