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

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Message-ID: <4d063dde-c710-44e3-31b9-8fb6b7e1d952@gmail.com>
Date:   Sat, 9 Jan 2021 20:37:17 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Chaitanya Kulkarni <Chaitanya.Kulkarni@....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 09/01/2021 20:09, Chaitanya Kulkarni wrote:
> 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.

I expect you won't find any, but such little things can pile up
into a not-easy-to-spot overhead over time.

In any case, I don't think this requires performance justification
because it neither makes it less safe or uglier. Those iov_iter*()
are there to handle initialisation, that's a part of the iter API.

>> 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;
> 

-- 
Pavel Begunkov