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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4792b04e03bsm99035395e9.7.2025.12.07.02.37.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 07 Dec 2025 02:37:57 -0800 (PST) Date: Sun, 7 Dec 2025 10:37:55 +0000 From: David Laight To: Mateusz Guzik Cc: Matthew Wilcox , oleg@redhat.com, brauner@kernel.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, linux-mm@kvack.org Subject: Re: [PATCH v3 2/2] pid: only take pidmap_lock once on alloc Message-ID: <20251207103755.70be2f89@pumpkin> In-Reply-To: References: <20251206131955.780557-1-mjguzik@gmail.com> <20251206131955.780557-3-mjguzik@gmail.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 3F6F7180003 X-Stat-Signature: n61th8au8fat4zhhaqn43uhmprabwo4g X-Rspam-User: X-HE-Tag: 1765103879-534321 X-HE-Meta: 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 1dGxjH20 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sun, 7 Dec 2025 10:21:35 +0100 Mateusz Guzik wrote: > On Sun, Dec 7, 2025 at 8:21=E2=80=AFAM Matthew Wilcox wrote: > > > > On Sat, Dec 06, 2025 at 02:19:55PM +0100, Mateusz Guzik wrote: =20 > > > - if (!pid) > > > + if (unlikely(!pid)) =20 > > > > Does this change anything? I was under the impression that gcc already > > treats !p as unlikely. =20 >=20 > I don't know if gcc is guaranteed to act like that, most of my > experience is with clang which was rather inconsistent about it. >=20 If nothing else unlikely() are information for the human (and other) reader. In my experience, a simple if () is always coded as a forwards jump around the controlled code - regardless of any likely/unlikely annotation. Similarly 'if () continue/break' is an immediate jump to the loop end code. You need to add a non-empty 'else' branch, or something before the continue/break for the un/likely to have an effect (an asm() comment will d= o). While some instruction sets have explicit 'branch expected taken' markers others (like x86 - except P6) don't. Similarly some cpu predict backwards branches taken if their is nothing in the branch predictor, others (including some x86) just use the relevant branch predictor 'slot' without regard for the actual branch address. Then there is the branch prediction done by the instruction fetch/decode unit, that has a tendency to use markers on the cache line - so doesn't necessarily even check it is a branch instruction! Basically the coder doesn't have much control at all on many cpu. Oh, and using un/likely() can make the code worse. David =20