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Date:   Mon, 18 Jul 2022 12:42:43 +0200
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To:     Vaibhav Jain <vaibhav@linux.ibm.com>, linux-kernel@vger.kernel.org
Cc:     "Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Muchun Song <songmuchun@bytedance.com>
References: <20220601163243.3806231-1-vaibhav@linux.ibm.com>
 <05623846-03c7-89f1-e1dd-0ee23723c7e9@redhat.com>
 <87r13p2jju.fsf@vajain21.in.ibm.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH] resource: re-factor page_is_ram()
In-Reply-To: <87r13p2jju.fsf@vajain21.in.ibm.com>
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[sorry for the late reply]

> 
> 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.

We usually don't care about such micro-optimizations unless you can
showcase actual performance numbers. Otherwise we'd have constant,
unnecessary code-churn all over the place.

Most probably, all that list walking dominates the runtime either way.

Feel free to proof me wrong ;)

> >>
>> 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.
> 

I'm not convinced, but I don't care enough to object. I'll add more
review feedback to the patch.

-- 
Thanks,

David / dhildenb