linux-kernel - Re: [PATCH] fs: optimise kiocb_set_rw

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Date:   Sun, 2 Aug 2020 11:33:37 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, Matthew Wilcox <willy@...radead.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: optimise kiocb_set_rw_flags()

On 01/08/2020 20:01, Jens Axboe wrote:
> On 8/1/20 9:37 AM, Matthew Wilcox wrote:
>> On Sat, Aug 01, 2020 at 01:36:33PM +0300, Pavel Begunkov wrote:
>>> Use a local var to collect flags in kiocb_set_rw_flags(). That spares
>>> some memory writes and allows to replace most of the jumps with MOVEcc.
>>>
>>> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
>>
>> Reviewed-by: Matthew Wilcox (Oracle) <willy@...radead.org>
>>
>> If you want to improve the codegen here further, I would suggest that
>> renumbering the IOCB flags to match the RWF flags would lead to better
>> codegen (can't do it the other way around; RWF flags are userspace ABI,
>> IOCB flags are not).  iocb_flags() probably doesn't get any worse because
>> the IOCB_ flags don't have the same numbers as the O_ bits (which differ
>> by arch anyway).
> 
> Yeah that's not a bad idea, would kill a lot of branches.

Is that common here to do so? I've done this for io_uring flags a while
ago, but left RWF alone at the time, reluctant to check for possible
complications (e.g. bit magic).

-- 
Pavel Begunkov