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(p200300cbc70574006b3aa74abd53a018.dip0.t-ipconnect.de. [2003:cb:c705:7400:6b3a:a74a:bd53:a018]) by smtp.gmail.com with ESMTPSA id id15-20020a05600ca18f00b003a31f1edfa7sm1226229wmb.41.2022.07.18.03.42.44 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 18 Jul 2022 03:42:44 -0700 (PDT) Message-ID: <096eec91-89a5-6068-acd3-c457d3ebd4d1@redhat.com> Date: Mon, 18 Jul 2022 12:42:43 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 Content-Language: en-US To: Vaibhav Jain , linux-kernel@vger.kernel.org Cc: "Aneesh Kumar K . V" , Andrew Morton , Dan Williams , Miaohe Lin , Muchun Song References: <20220601163243.3806231-1-vaibhav@linux.ibm.com> <05623846-03c7-89f1-e1dd-0ee23723c7e9@redhat.com> <87r13p2jju.fsf@vajain21.in.ibm.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH] resource: re-factor page_is_ram() In-Reply-To: <87r13p2jju.fsf@vajain21.in.ibm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org [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 > 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 > 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 > 0x0000000000001005 <+5>: shl $0xc,%rdi > 0x0000000000001009 <+9>: lea 0xfff(%rdi),%rsi > 0x0000000000001010 <+16>: cmp %rsi,%rdi > 0x0000000000001013 <+19>: jae 0x1064 > 0x0000000000001015 <+21>: sub $0x40,%rsp > 0x0000000000001019 <+25>: xor %ecx,%ecx > 0x000000000000101b <+27>: mov $0x81000200,%edx > 0x0000000000001020 <+32>: mov %rsp,%r8 > 0x0000000000001023 <+35>: call 0x7e0 > 0x0000000000001028 <+40>: test %eax,%eax > 0x000000000000102a <+42>: jne 0x105a > 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 > 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