Re: [PATCH] resource: re-factor page_is_ram()

[Vaibhav Jain <vaibhav@linux.ibm.com>]

Hi David,

Thanks for looking into this patch,

David Hildenbrand <david@redhat.com> writes:

> On 01.06.22 18:32, Vaibhav Jain wrote:
>> Presently page_is_ram() relies on walk_system_ram_range() that performs a walk
>> on kernel iomem resources hierarchy with a dummy callback __is_ram(). Before
>> calling find_next_iomem_res(), walk_system_ram_range() does some book-keeping
>> which can be avoided for page_is_ram() use-case.
>> 
>> Hence this patch proposes to update page_is_ram() to directly call
>> find_next_iomem_res() with minimal book-keeping needed.
>
> I consider the code harder to get compared to just reusing the
> more-generic and expressive walk_system_ram_range()
>
> It somehow feels like we're duplicating the code here just to optimize
> out a handful of instructions.

The only reason for existence of dummy callback __is_ram() is for
page_is_ram() to be able to use walk_system_ram_range(). For
page_is_ram() usecase what walk_system_ram_range() essentially does is
to iterate over find_next_iomem_res() and call __is_ram() which is not
really needed to page_is_ram().

The improvement to the gcc (v12.1.1) generated code (x86_64) for
page_is_ram is quite evident.

With the patch:
   0x0000000000000920 <+0>:	call   0x925 <page_is_ram+5>
   0x0000000000000925 <+5>:	shl    $0xc,%rdi
   0x0000000000000929 <+9>:	xor    %r8d,%r8d
   0x000000000000092c <+12>:	xor    %ecx,%ecx
   0x000000000000092e <+14>:	mov    $0x81000200,%edx
   0x0000000000000933 <+19>:	lea    0x1(%rdi),%rsi
   0x0000000000000937 <+23>:	call   0x7e0 <find_next_iomem_res>
   0x000000000000093c <+28>:	test   %eax,%eax
   0x000000000000093e <+30>:	sete   %al
   0x0000000000000941 <+33>:	movzbl %al,%eax
   0x0000000000000944 <+36>:	ret
   0x0000000000000945 <+37>:	int3

Without the patch:
   0x0000000000001000 <+0>:	call   0x1005 <page_is_ram+5>
   0x0000000000001005 <+5>:	shl    $0xc,%rdi
   0x0000000000001009 <+9>:	lea    0xfff(%rdi),%rsi
   0x0000000000001010 <+16>:	cmp    %rsi,%rdi
   0x0000000000001013 <+19>:	jae    0x1064 <page_is_ram+100>
   0x0000000000001015 <+21>:	sub    $0x40,%rsp
   0x0000000000001019 <+25>:	xor    %ecx,%ecx
   0x000000000000101b <+27>:	mov    $0x81000200,%edx
   0x0000000000001020 <+32>:	mov    %rsp,%r8
   0x0000000000001023 <+35>:	call   0x7e0 <find_next_iomem_res>
   0x0000000000001028 <+40>:	test   %eax,%eax
   0x000000000000102a <+42>:	jne    0x105a <page_is_ram+90>
   0x000000000000102c <+44>:	mov    (%rsp),%rax
   0x0000000000001030 <+48>:	mov    $0x1,%ecx
   0x0000000000001035 <+53>:	lea    0xfff(%rax),%rdx
   0x000000000000103c <+60>:	mov    0x8(%rsp),%rax
   0x0000000000001041 <+65>:	shr    $0xc,%rdx
   0x0000000000001045 <+69>:	add    $0x1,%rax
   0x0000000000001049 <+73>:	shr    $0xc,%rax
   0x000000000000104d <+77>:	cmp    %rax,%rdx
   0x0000000000001050 <+80>:	jae    0x105a <page_is_ram+90>
   0x0000000000001052 <+82>:	mov    %ecx,%eax
   0x0000000000001054 <+84>:	add    $0x40,%rsp
   0x0000000000001058 <+88>:	ret
   0x0000000000001059 <+89>:	int3
   0x000000000000105a <+90>:	xor    %ecx,%ecx
   0x000000000000105c <+92>:	add    $0x40,%rsp
   0x0000000000001060 <+96>:	mov    %ecx,%eax
   0x0000000000001062 <+98>:	ret
   0x0000000000001063 <+99>:	int3
   0x0000000000001064 <+100>:	xor    %eax,%eax
   0x0000000000001066 <+102>:	ret
   0x0000000000001067 <+103>:	int3

Looking at the disassembly above, gcc has inlined both walk_system_ram_range()
and __is_ram() in page_is_ram(). This ends up in page_is_ram() calling
find_next_iomem_res() directly anyways with bunch of book-keeping
afterwards which can be avoided.

>
> If it doesn't make the code easier to read (at least for me), why do we
> care?
IMHO, calling find_next_iomem_res() from page_is_ram() instead of
calling walk_system_ram_range() makes it easy to trace the path of
page_is_ram(). Also the dummy callback makes the code flow seems strange
initially.

-- 
Cheers
~ Vaibhav