linux-kernel - Re: [PATCH] mm: remove unneeded _

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Date:   Tue, 8 Aug 2023 14:10:43 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: remove unneeded __meminit annotation

On 2023/8/8 11:26, Matthew Wilcox wrote:
> On Tue, Aug 08, 2023 at 04:16:57AM +0100, Matthew Wilcox wrote:
>> On Tue, Aug 08, 2023 at 11:08:36AM +0800, Miaohe Lin wrote:
>>> On 2023/8/8 10:31, Matthew Wilcox wrote:
>>>> On Tue, Aug 08, 2023 at 09:58:31AM +0800, Miaohe Lin wrote:
>>>>> kswapd_stop() and kcompactd_stop() are only called when MEMORY_HOTREMOVE
>>>>> is enabled. So wrap them under CONFIG_MEMORY_HOTREMOVE and further remove
>>>>> __meminit annotation. No functional change intended.
>>>>
>>>> I don't understand why this is an improvement.  If CONFIG_MEMORY_HOTREMOVE
>>>> is disabled, the linker drops this section (... right?)  If it's enabled,
>>>
>>> When CONFIG_MEMORY_HOTREMOVE is disabled, without this patch:
>>>
>>> size mm/compaction.o
>>>    text	   data	    bss	    dec	    hex	filename
>>>  103164	  30873	      0	 134037	  20b95	mm/compaction.o
>>>
>>> size mm/vmscan.o
>>>    text	   data	    bss	    dec	    hex	filename
>>>  158775	  49612	     64	 208451	  32e43	mm/vmscan.o
>>>
>>> while with this patch:
>>>
>>> size mm/compaction.o
>>>    text	   data	    bss	    dec	    hex	filename
>>>  102915	  30865	      0	 133780	  20a94	mm/compaction.o
>>>
>>> size mm/vmscan.o
>>>    text	   data	    bss	    dec	    hex	filename
>>>  158534	  49604	     64	 208202	  32d4a	mm/vmscan.o
>>>
>>> We can reduce each .o by ~250 bytes.
>>
>> But this is before the linker step!  That will be where the meminit
>> sections get dropped.  Assuming they are; I haven't verified.  You need
>> to compare before/after of the vmlinux, not the individual .o files.

When CONFIG_MEMORY_HOTREMOVE is disabled, without patch:

size vmlinux
   text	   data	    bss	    dec	    hex	filename
70648396	21668938	17985540	110302874	693169a	vmlinux

while with patch:

size vmlinux
   text	   data	    bss	    dec	    hex	filename
70648128	21668938	17985540	110302606	693158e	vmlinux

vmlinux is still reduced by 268 bytes.

> 
> Ah, found it:
> 
> #if defined(CONFIG_MEMORY_HOTPLUG)
> #define MEM_KEEP(sec)    *(.mem##sec)
> #define MEM_DISCARD(sec)
> #else
> #define MEM_KEEP(sec)
> #define MEM_DISCARD(sec) *(.mem##sec)
> #endif
> 
> in include/asm-generic/vmlinux.lds.h

When CONFIG_MEMORY_HOTREMOVE is disabled, without this patch, kswapd_stop() and kcompactd_stop()
are dropped after system init.

But no insist in this patch.

Thanks for your comment.