linux-kernel - Re: [PATCH] direct-io: Deinline dio_zero

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Message-ID: <CACVXFVPnUY10wyezv4JudXAFyrZtBzk5UYAevZEOc-ht=TupMg@mail.gmail.com>
Date:	Mon, 14 Mar 2016 09:07:12 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>, Jens Axboe <axboe@...com>,
	Christoph Hellwig <hch@....de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] direct-io: Deinline dio_zero_block, save 2684 bytes

On Mon, Mar 14, 2016 at 5:36 AM, Denys Vlasenko
<vda.linux@...glemail.com> wrote:
> On Sun, Mar 13, 2016 at 10:00 PM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
>> This function compiles to 2684 bytes, 2 callsites
>>
>>    text    data     bss     dec     hex filename
>>    9655      16       0    9671    25c7 direct-io.o.before2
>>    9559      16       0    9575    2567 direct-io.o
>
> Al, you undoubtedly noticed the discrepancy between
> "save 2684 bytes" claim in the commit description
> and the above data, which shows no such thing.
>
> Sorry, I was too quick to send the mail to notice it :(
>
> Further investigation had shown that it's my particular version of gcc
> deciding to deinline the function even before the patch.
> Other version of gcc, on a different machine was running
> large inlining search script, did not do that, and saw the 2684 bytes
> of savings.
>
> tl;dr: the patch is correct, my "size" printout wasn't.

Not sure the patch is correct, please see the commit for
do_blockdev_direct_IO():

 * NOTE: if you pass "sdio" to anything by pointer make sure that function
 * is always inlined. Otherwise gcc is unable to split the structure into
 * individual fields and will generate much worse code. This is important
 * for the whole file.



thanks,
Ming Lei