linux-kernel - Re: [PATCH] target/file: don't zero iter before iov_iter

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Message-ID: <BYAPR04MB4965F4DCF59E5225CF17322D86AD0@BYAPR04MB4965.namprd04.prod.outlook.com>
Date:   Sat, 9 Jan 2021 20:09:15 +0000
From:   Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>
To:     Pavel Begunkov <asml.silence@...il.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>
CC:     "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "target-devel@...r.kernel.org" <target-devel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] target/file: don't zero iter before iov_iter_bvec

On 1/9/21 07:59, Pavel Begunkov wrote:
> iov_iter_bvec() initialises iterators well, no need to pre-zero it
> beforehand as done in fd_execute_rw_aio(). Compilers can't optimise it
> out and generate extra code for that (confirmed with assembly).
It will be great if we can quantify this optimization with the actual
performance
numbers.
> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> ---
>  drivers/target/target_core_file.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/target/target_core_file.c b/drivers/target/target_core_file.c
> index cce455929778..5a66854def95 100644
> --- a/drivers/target/target_core_file.c
> +++ b/drivers/target/target_core_file.c
> @@ -267,7 +267,7 @@ fd_execute_rw_aio(struct se_cmd *cmd, struct scatterlist *sgl, u32 sgl_nents,
>  	struct fd_dev *fd_dev = FD_DEV(dev);
>  	struct file *file = fd_dev->fd_file;
>  	struct target_core_file_cmd *aio_cmd;
> -	struct iov_iter iter = {};
> +	struct iov_iter iter;
>  	struct scatterlist *sg;
>  	ssize_t len = 0;
>  	int ret = 0, i;