Date:   Mon, 21 Feb 2022 18:09:35 +0100
From:   Bodo Stroesser <bostroesser@...il.com>
To:     Guixin Liu <kanie@...ux.alibaba.com>, gregkh@...uxfoundation.org,
        martin.petersen@...cle.com
Cc:     linux-scsi@...r.kernel.org, target-devel@...r.kernel.org,
        linux-kernel@...r.kernel.org, xiaoguang.wang@...ux.alibaba.com,
        xlpang@...ux.alibaba.com
Subject: Re: [PATCH 2/2] scsi:target:tcmu: reduce once copy by using uio ioctl

Liu,

generally I like ideas to speed up tcmu.

OTOH, since Andy Grover implemented tcmu based on uio device, we are
restricted to what uio offers. With today's knowledge I think we would
not use the uio device in tcmu again, but switching away from uio now
would break existing userspace SW.

Before we start thinking how the same performance gain could be reached
without a change in uio device, please let me know why you use file
backend on top of tcmu instead of target_core_file kernel module?
Wouldn't target_core_file be even faster for your purpose?

Bodo



On 17.02.22 03:29, Guixin Liu wrote:
> Currently the data needs to be copied twice between sg, tcmu data area and
> userspace buffer if backstore holds its own userspace buffer, then we can
> use uio ioctl to copy data between sg and userspace buffer directly to
> bypass data area to improve performance.
> 
> Use tcm_loop and tcmu(backstore is file) to evaluate performance,
> fio job: fio -filename=/dev/sdb  -direct=1 -size=2G -name=1 -thread
> -runtime=60 -time_based -rw=randread -numjobs=16 -iodepth=16 -bs=128k
> 
> Without this patch:
>      READ: bw=3539MiB/s (3711MB/s), 207MiB/s-233MiB/s (217MB/s-244MB/s),
> io=104GiB (111GB), run=30001-30002msec
> 
> With this patch:
>      READ: bw=4420MiB/s (4634MB/s), 274MiB/s-278MiB/s (287MB/s-291MB/s),
> io=259GiB (278GB), run=60001-60002msec
> 
> Reviewed-by: Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>
> Signed-off-by: Guixin Liu <kanie@...ux.alibaba.com>
> ---
>   drivers/target/target_core_user.c     | 171 +++++++++++++++++++++++++++++-----
>   include/uapi/linux/target_core_user.h |   9 ++
>   2 files changed, 157 insertions(+), 23 deletions(-)
> 
> diff --git a/drivers/target/target_core_user.c b/drivers/target/target_core_user.c
> index 7b2a89a..afea088 100644
> --- a/drivers/target/target_core_user.c
> +++ b/drivers/target/target_core_user.c
> @@ -122,6 +122,7 @@ struct tcmu_dev {
>   #define TCMU_DEV_BIT_BLOCKED 2
>   #define TCMU_DEV_BIT_TMR_NOTIFY 3
>   #define TCMU_DEV_BIT_PLUGGED 4
> +#define TCMU_DEV_BIT_BYPASS_DATA_AREA 5
>   	unsigned long flags;
>   
>   	struct uio_info uio_info;
> @@ -642,12 +643,17 @@ static struct tcmu_cmd *tcmu_alloc_cmd(struct se_cmd *se_cmd)
>   	tcmu_cmd->se_cmd = se_cmd;
>   	tcmu_cmd->tcmu_dev = udev;
>   
> -	tcmu_cmd_set_block_cnts(tcmu_cmd);
> -	tcmu_cmd->dbi = kcalloc(tcmu_cmd->dbi_cnt, sizeof(uint32_t),
> -				GFP_NOIO);
> -	if (!tcmu_cmd->dbi) {
> -		kmem_cache_free(tcmu_cmd_cache, tcmu_cmd);
> -		return NULL;
> +	if (!test_bit(TCMU_DEV_BIT_BYPASS_DATA_AREA, &udev->flags)) {
> +		tcmu_cmd_set_block_cnts(tcmu_cmd);
> +		tcmu_cmd->dbi = kcalloc(tcmu_cmd->dbi_cnt, sizeof(uint32_t),
> +					GFP_NOIO);
> +		if (!tcmu_cmd->dbi) {
> +			kmem_cache_free(tcmu_cmd_cache, tcmu_cmd);
> +			return NULL;
> +		}
> +	} else {
> +		tcmu_cmd->dbi_cnt = 0;
> +		tcmu_cmd->dbi = NULL;
>   	}
>   
>   	return tcmu_cmd;
> @@ -1093,16 +1099,18 @@ static int queue_cmd_ring(struct tcmu_cmd *tcmu_cmd, sense_reason_t *scsi_err)
>   	tcmu_cmd_reset_dbi_cur(tcmu_cmd);
>   	iov = &entry->req.iov[0];
>   
> -	if (se_cmd->data_direction == DMA_TO_DEVICE ||
> -	    se_cmd->se_cmd_flags & SCF_BIDI)
> -		scatter_data_area(udev, tcmu_cmd, &iov);
> -	else
> -		tcmu_setup_iovs(udev, tcmu_cmd, &iov, se_cmd->data_length);
> -
> +	if (!test_bit(TCMU_DEV_BIT_BYPASS_DATA_AREA, &udev->flags)) {
> +		if (se_cmd->data_direction == DMA_TO_DEVICE ||
> +		se_cmd->se_cmd_flags & SCF_BIDI)
> +			scatter_data_area(udev, tcmu_cmd, &iov);
> +		else
> +			tcmu_setup_iovs(udev, tcmu_cmd, &iov, se_cmd->data_length);
> +	}
>   	entry->req.iov_cnt = iov_cnt - iov_bidi_cnt;
>   
>   	/* Handle BIDI commands */
> -	if (se_cmd->se_cmd_flags & SCF_BIDI) {
> +	if ((se_cmd->se_cmd_flags & SCF_BIDI)
> +		&& !test_bit(TCMU_DEV_BIT_BYPASS_DATA_AREA, &udev->flags)) {
>   		iov++;
>   		tcmu_setup_iovs(udev, tcmu_cmd, &iov, tcmu_cmd->data_len_bidi);
>   		entry->req.iov_bidi_cnt = iov_bidi_cnt;
> @@ -1366,16 +1374,19 @@ static bool tcmu_handle_completion(struct tcmu_cmd *cmd,
>   		else
>   			se_cmd->se_cmd_flags |= SCF_TREAT_READ_AS_NORMAL;
>   	}
> -	if (se_cmd->se_cmd_flags & SCF_BIDI) {
> -		/* Get Data-In buffer before clean up */
> -		gather_data_area(udev, cmd, true, read_len);
> -	} else if (se_cmd->data_direction == DMA_FROM_DEVICE) {
> -		gather_data_area(udev, cmd, false, read_len);
> -	} else if (se_cmd->data_direction == DMA_TO_DEVICE) {
> -		/* TODO: */
> -	} else if (se_cmd->data_direction != DMA_NONE) {
> -		pr_warn("TCMU: data direction was %d!\n",
> -			se_cmd->data_direction);
> +
> +	if (!test_bit(TCMU_DEV_BIT_BYPASS_DATA_AREA, &udev->flags)) {
> +		if (se_cmd->se_cmd_flags & SCF_BIDI) {
> +			/* Get Data-In buffer before clean up */
> +			gather_data_area(udev, cmd, true, read_len);
> +		} else if (se_cmd->data_direction == DMA_FROM_DEVICE) {
> +			gather_data_area(udev, cmd, false, read_len);
> +		} else if (se_cmd->data_direction == DMA_TO_DEVICE) {
> +			/* TODO: */
> +		} else if (se_cmd->data_direction != DMA_NONE) {
> +			pr_warn("TCMU: data direction was %d!\n",
> +				se_cmd->data_direction);
> +		}
>   	}
>   
>   done:
> @@ -1973,6 +1984,84 @@ static int tcmu_release(struct uio_info *info, struct inode *inode)
>   	return 0;
>   }
>   
> +long tcmu_ioctl_copy_between_sgl_and_iovec(struct tcmu_cmd *tcmu_cmd,
> +			struct iovec __user *uiovec,
> +			unsigned long vcnt,
> +			bool is_copy_to_sgl)
> +{
> +	struct iovec iovstack[UIO_FASTIOV];
> +	struct iovec *iov = iovstack;
> +	struct iov_iter iter;
> +	ssize_t ret;
> +	struct se_cmd *se_cmd = tcmu_cmd->se_cmd;
> +	struct scatterlist *data_sg, *sg;
> +	int i;
> +	unsigned int data_nents;
> +	long copy_ret = 0;
> +
> +	if (se_cmd->se_cmd_flags & SCF_BIDI) {
> +		data_sg = se_cmd->t_bidi_data_sg;
> +		data_nents = se_cmd->t_bidi_data_nents;
> +	} else {
> +		data_sg = se_cmd->t_data_sg;
> +		data_nents = se_cmd->t_data_nents;
> +	}
> +
> +	ret = import_iovec(READ, uiovec, vcnt, ARRAY_SIZE(iovstack), &iov, &iter);
> +	if (ret < 0) {
> +		pr_err("import iovec failed.\n");
> +		return -EFAULT;
> +	}
> +
> +	for_each_sg(data_sg, sg, data_nents, i) {
> +		if (is_copy_to_sgl)
> +			ret = copy_page_from_iter(sg_page(sg), sg->offset, sg->length, &iter);
> +		else
> +			ret = copy_page_to_iter(sg_page(sg), sg->offset, sg->length, &iter);
> +		if (ret < 0) {
> +			pr_err("copy failed.\n");
> +			copy_ret = -EFAULT;
> +			break;
> +		}
> +	}
> +	kfree(iov);
> +	return copy_ret;
> +}
> +
> +long tcmu_ioctl(struct uio_info *info, unsigned int cmd, unsigned long arg)
> +{
> +	struct tcmu_dev *udev = container_of(info, struct tcmu_dev, uio_info);
> +	struct tcmu_data_xfer __user *uxfer = (struct tcmu_data_xfer __user *)arg;
> +	struct tcmu_data_xfer xfer;
> +	struct tcmu_cmd *tcmu_cmd;
> +
> +	if (!test_bit(TCMU_DEV_BIT_BYPASS_DATA_AREA, &udev->flags))
> +		return -EINVAL;
> +
> +	if (copy_from_user(&xfer, uxfer, sizeof(xfer)))
> +		return -EFAULT;
> +
> +	tcmu_cmd = xa_load(&udev->commands, xfer.cmd_id);
> +	if (!tcmu_cmd) {
> +		set_bit(TCMU_DEV_BIT_BROKEN, &udev->flags);
> +		return -EFAULT;
> +	}
> +
> +	if (test_bit(TCMU_CMD_BIT_EXPIRED, &tcmu_cmd->flags))
> +		return -EFAULT;
> +
> +	switch (cmd) {
> +	case TCMU_IOCTL_CMD_COPY_TO_SGL:
> +		return tcmu_ioctl_copy_between_sgl_and_iovec(tcmu_cmd, xfer.iovec,
> +							     xfer.iov_cnt, true);
> +	case TCMU_IOCTL_CMD_COPY_FROM_SGL:
> +		return tcmu_ioctl_copy_between_sgl_and_iovec(tcmu_cmd, xfer.iovec,
> +							     xfer.iov_cnt, false);
> +	default:
> +		return -EINVAL;
> +	}
> +}
> +
>   static int tcmu_init_genl_cmd_reply(struct tcmu_dev *udev, int cmd)
>   {
>   	struct tcmu_nl_cmd *nl_cmd = &udev->curr_nl_cmd;
> @@ -2230,6 +2319,7 @@ static int tcmu_configure_device(struct se_device *dev)
>   	info->mmap = tcmu_mmap;
>   	info->open = tcmu_open;
>   	info->release = tcmu_release;
> +	info->ioctl = tcmu_ioctl;
>   
>   	ret = uio_register_device(tcmu_root_device, info);
>   	if (ret)
> @@ -2838,6 +2928,40 @@ static ssize_t tcmu_nl_reply_supported_store(struct config_item *item,
>   }
>   CONFIGFS_ATTR(tcmu_, nl_reply_supported);
>   
> +static ssize_t tcmu_bypass_data_area_show(struct config_item *item, char *page)
> +{
> +	struct se_dev_attrib *da = container_of(to_config_group(item),
> +						struct se_dev_attrib, da_group);
> +	struct tcmu_dev *udev = TCMU_DEV(da->da_dev);
> +
> +	if (test_bit(TCMU_DEV_BIT_BYPASS_DATA_AREA, &udev->flags))
> +		return snprintf(page, PAGE_SIZE, "%s\n", "true");
> +	else
> +		return snprintf(page, PAGE_SIZE, "%s\n", "false");
> +}
> +
> +static ssize_t tcmu_bypass_data_area_store(struct config_item *item, const char *page,
> +					    size_t count)
> +{
> +	struct se_dev_attrib *da = container_of(to_config_group(item),
> +						struct se_dev_attrib, da_group);
> +	struct tcmu_dev *udev = TCMU_DEV(da->da_dev);
> +	bool bypass_data_area;
> +	int ret;
> +
> +	ret = strtobool(page, &bypass_data_area);
> +	if (ret < 0)
> +		return ret;
> +
> +	if (bypass_data_area)
> +		set_bit(TCMU_DEV_BIT_BYPASS_DATA_AREA, &udev->flags);
> +	else
> +		clear_bit(TCMU_DEV_BIT_BYPASS_DATA_AREA, &udev->flags);
> +
> +	return count;
> +}
> +CONFIGFS_ATTR(tcmu_, bypass_data_area);
> +
>   static ssize_t tcmu_emulate_write_cache_show(struct config_item *item,
>   					     char *page)
>   {
> @@ -3069,6 +3193,7 @@ static ssize_t tcmu_free_kept_buf_store(struct config_item *item, const char *pa
>   	&tcmu_attr_emulate_write_cache,
>   	&tcmu_attr_tmr_notification,
>   	&tcmu_attr_nl_reply_supported,
> +	&tcmu_attr_bypass_data_area,
>   	NULL,
>   };
>   
> diff --git a/include/uapi/linux/target_core_user.h b/include/uapi/linux/target_core_user.h
> index 27ace51..c02a45e 100644
> --- a/include/uapi/linux/target_core_user.h
> +++ b/include/uapi/linux/target_core_user.h
> @@ -185,4 +185,13 @@ enum tcmu_genl_attr {
>   };
>   #define TCMU_ATTR_MAX (__TCMU_ATTR_MAX - 1)
>   
> +struct tcmu_data_xfer {
> +	unsigned short cmd_id;
> +	unsigned long iov_cnt;
> +	struct iovec __user *iovec;
> +};
> +
> +#define TCMU_IOCTL_CMD_COPY_TO_SGL      _IOW('T', 0xe0, struct tcmu_data_xfer)
> +#define TCMU_IOCTL_CMD_COPY_FROM_SGL    _IOW('T', 0xe1, struct tcmu_data_xfer)
> +
>   #endif

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