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[209.85.160.175]) by smtp.gmail.com with ESMTPSA id f1-20020a05620a280100b006eef13ef4c8sm20770409qkp.94.2023.01.17.09.36.21 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 17 Jan 2023 09:36:21 -0800 (PST) Received: by mail-qt1-f175.google.com with SMTP id r15so4574994qtx.6 for ; Tue, 17 Jan 2023 09:36:21 -0800 (PST) X-Received: by 2002:a05:620a:99d:b0:705:efa8:524c with SMTP id x29-20020a05620a099d00b00705efa8524cmr178735qkx.594.1673976555002; Tue, 17 Jan 2023 09:29:15 -0800 (PST) MIME-Version: 1.0 References: <20230111123736.20025-1-kirill.shutemov@linux.intel.com> <20230111123736.20025-9-kirill.shutemov@linux.intel.com> <20230117135703.voaumisreld7crfb@box> In-Reply-To: From: Linus Torvalds Date: Tue, 17 Jan 2023 09:28:59 -0800 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCHv14 08/17] x86/mm: Reduce untagged_addr() overhead until the first LAM user To: Peter Zijlstra Cc: "Kirill A. Shutemov" , Dave Hansen , Andy Lutomirski , x86@kernel.org, Kostya Serebryany , Andrey Ryabinin , Andrey Konovalov , Alexander Potapenko , Taras Madan , Dmitry Vyukov , "H . J . Lu" , Andi Kleen , Rick Edgecombe , Bharata B Rao , Jacob Pan , Ashok Raj , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Sami Tolvanen , ndesaulniers@google.com, joao@overdrivepizza.com Content-Type: text/plain; charset="UTF-8" X-Rspamd-Queue-Id: AFB67C0011 X-Stat-Signature: yyhryffff14f34ks7ej1b5m5pdqownod X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1673976982-612017 X-HE-Meta: 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 32GetGlJ K5rSaFH6astTkKvkKT/10AmOkyhbV6Nq1JPwGadW/L3ZMWqhud9tzUYq7uru1xDa9YvjqQTPaKnhVEsWk9FaEpAg0MOOnZ0AERJ9wKKLbXaoFqhvhifL6TNVk0c5K0hIDvYXxPVvOA1Z7TzB9zS1t2wOgbqZ5a70kBVqhEuvF2LB8Cf1IK63ofm+OGmzLIRT0P/9Ir+rxy4UNWjPrE4aGrjc2UA9bohIc1PnDtKe1yxSrJe7m1jkKutrmq6ej7FobuPUNmOz8gl25BS78oneL1q6d4PF2hEAG9Ow4kGxeWrtdyL46T+mWMA1sLj+RuL8Rv1iekbluh9Skobw/t8XZBO3cRR6+oG97CfaJKAz6XSz1seHJGLyzvVywN+PrdhiO65snN12sxxRrFUyCflx5Npc+Tg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000003, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Jan 17, 2023 at 9:18 AM Linus Torvalds wrote: > > The reason clang seems to generate saner code is that clang seems to > largely ignore the whole "__builtin_expect()", at least not to the > point where it tries to make the unlikely case be out-of-line. Side note: that's not something new or unusual. It's been the case since I started testing clang - we have several code-paths where we use "unlikely()" to try to get very unlikely cases to be out-of-line, and clang just mostly ignores it, or treats it as a very weak hint. I think the only way to get clang to treat it as a *strong* hint is to use PGO. And in this case it actually made code generation look better, probably because this particular use of static_branch_likely() is a bit confused about which side should be the preferred one. It's using the static branch to make the old case not have the masked load, but then it's saying that the new case is the likely one. So clang ignoring the likely() hint is probably the right thing here, and then the wrong thing in some other places. Linus