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AJvYcCUsyBASp8kZUt6/mdnStzjPsIem5MX4fPtEz7Vu8Kc+FTLNbhMR5C9IlAQeZ3pM1u7WSrwAUFsWeA==@kvack.org X-Gm-Message-State: AOJu0Yycq5iKcNix5v93NfpWXR6LFjY5K60H+J4Ir2DXRJxDFraUsjba CDzXYchPRXkv5Em6yxu8nYhXkLzShWJNy2R+yCJXDTrljJI//5h7fwqVViE6AykBwEWbnqqQy/u 6i9Wo0fYTBFOZNNsMPWXq/J938XTh1+LJ0NPDXQ== X-Gm-Gg: ASbGncsBDjvTLRZxMbp4iUe2nGco0+6+/VspBPSYTALBfv21Ej2PJQKlhxFo7VauIuI YoJbf+d6SozYTe98ydmrGPdWeHW5BfO2aig== X-Google-Smtp-Source: AGHT+IGB6//Aa/9qdBUs97OqLI01wh9FfTforqMFQJWtL6U0HM26bdGgt3fd2jPfYC1+ZrQOgriT60mCRL7KIvudlxQ= X-Received: by 2002:a05:6512:3e01:b0:540:26f2:3e18 with SMTP id 2adb3069b0e04-5402a60c377mr883807e87.49.1733922255058; Wed, 11 Dec 2024 05:04:15 -0800 (PST) MIME-Version: 1.0 References: <20241210160556.2341497-1-arnd@kernel.org> <20241210160556.2341497-2-arnd@kernel.org> In-Reply-To: <20241210160556.2341497-2-arnd@kernel.org> From: Linus Walleij Date: Wed, 11 Dec 2024 14:04:03 +0100 Message-ID: Subject: Re: [PATCH 1/4] ARM: Disable jump-label on PREEMPT_RT To: Arnd Bergmann Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Arnd Bergmann , linux-mm@kvack.org, linux-rt-devel@lists.linux.dev, Ard Biesheuvel , Clark Williams , Jason Baron , Josh Poimboeuf , Mark Rutland , Matthew Wilcox , Peter Zijlstra , Russell King , Sebastian Andrzej Siewior , Steven Rostedt , Thomas Gleixner Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: A04231A0007 X-Stat-Signature: csk3ps5x5fdr4so9g9imb4em8kmbiw3x X-Rspam-User: X-Rspamd-Server: rspam09 X-HE-Tag: 1733922232-868975 X-HE-Meta: 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 VVkzQ1FC UQylcvhj8lfP+N/Oo+C5aOTB2HBEki8qZ5cUBHdlf0XHidqS3adxubYJZwUEmT7ZTLUCnuSZXTobIHLfM510ulyTdR7PTmPbMoKOkpnTZBSQ2jobyPJpbQRGKp1dVIG0dxCfuh7RfDdQWcxOdqlMidHBLK9CeVp1GJ4F7x8LCJDqjWigA6EQh3AAR8Ik2kzoonO/o4cxK9OsfRrZiE2edgMKF/Uk/NQ9t5TBG6oKmPxAae1rzSsz+YEsLpcdbvZ0L6tfcTAtSaCvp23MK2BQJqqJEU9gnfg8JIgeRzrDzJon7IPDm3nqMyujdpVCxKVD1tCVPPs7n/0GfIiJ4muc0bCSVgWSewdK2DpGE1OVOOdfdy96sJioWSdHZz5tkTVdt64U1 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000007, 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 Tue, Dec 10, 2024 at 5:06=E2=80=AFPM Arnd Bergmann wro= te: > From: Thomas Gleixner > > jump-labels are used to efficiently switch between two possible code > paths. To achieve this, stop_machine() is used to keep the CPU in a > known state while the opcode is modified. The usage of stop_machine() > here leads to large latency spikes which can be observed on PREEMPT_RT. > > Jump labels may change the target during runtime and are not restricted > to debug or "configuration/ setup" part of a PREEMPT_RT system where > high latencies could be defined as acceptable. > > On 64-bit Arm, it is possible to use jump labels without the > stop_machine() call, which architecturally provides a way to atomically > change one 32-bit instruction word while keeping maintaining consistency, > but this is not generally the case on 32-bit, in particular in thumb2 > mode. > > Disable jump-label support on a PREEMPT_RT system when SMP is enabled. > > [bigeasy: Patch description.] > [arnd: add !SMP case, extend changelog] > > Signed-off-by: Thomas Gleixner > Signed-off-by: Sebastian Andrzej Siewior > Cc: Mark Rutland > Cc: Ard Biesheuvel > Link: https://lkml.kernel.org/r/20220613182447.112191-2-bigeasy@linutroni= x.de > Signed-off-by: Arnd Bergmann Makes sense. Reviewed-by: Linus Walleij Yours, Linus Walleij