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

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Date:   Fri, 17 Jan 2020 04:23:55 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     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 17/01/2020 04:21, Matthew Wilcox wrote:
> On Fri, Jan 17, 2020 at 04:16:41AM +0300, Pavel Begunkov wrote:
>> kiocb_set_rw_flags() generates a poor code with several memory writes
>> and a lot of jumps. Help compilers to optimise it.
>>
>> Tested with gcc 9.2 on x64-86, and as a result, it its output now is a
>> plain code without jumps accumulating in a register before a memory
>> write.
> 
> Nice!
> 
>>  static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
>>  {
>> +	int kiocb_flags = 0;
>> +
>>  	if (unlikely(flags & ~RWF_SUPPORTED))
>>  		return -EOPNOTSUPP;
>>  
>>  	if (flags & RWF_NOWAIT) {
>>  		if (!(ki->ki_filp->f_mode & FMODE_NOWAIT))
>>  			return -EOPNOTSUPP;
>> -		ki->ki_flags |= IOCB_NOWAIT;
>> +		kiocb_flags |= IOCB_NOWAIT;
>>  	}
>>  	if (flags & RWF_HIPRI)
>> -		ki->ki_flags |= IOCB_HIPRI;
>> +		kiocb_flags |= IOCB_HIPRI;
>>  	if (flags & RWF_DSYNC)
>> -		ki->ki_flags |= IOCB_DSYNC;
>> +		kiocb_flags |= IOCB_DSYNC;
>>  	if (flags & RWF_SYNC)
>> -		ki->ki_flags |= (IOCB_DSYNC | IOCB_SYNC);
>> +		kiocb_flags |= (IOCB_DSYNC | IOCB_SYNC);
>>  	if (flags & RWF_APPEND)
>> -		ki->ki_flags |= IOCB_APPEND;
>> +		kiocb_flags |= IOCB_APPEND;
>> +
>> +	if (kiocb_flags)
>> +		ki->ki_flags |= kiocb_flags;
>>  	return 0;
>>  }
> 
> Might it generate even better code to do ...

Good idea, thanks! I'll resend

> 
>  	int kiocb_flags = 0;
>  
> +	if (!flags)
> +		return 0;
>  	if (unlikely(flags & ~RWF_SUPPORTED))
>  		return -EOPNOTSUPP;
>  
> ...
> 
> -	if (kiocb_flags)
> -		ki->ki_flags |= kiocb_flags;
> +	ki->ki_flags |= kiocb_flags;
> 

-- 
Pavel Begunkov



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