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[209.85.208.44]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-ab6e4a31f41sm498852766b.144.2025.02.01.15.24.52 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 01 Feb 2025 15:24:53 -0800 (PST) Received: by mail-ed1-f44.google.com with SMTP id 4fb4d7f45d1cf-5dc0522475eso6466082a12.1 for ; Sat, 01 Feb 2025 15:24:52 -0800 (PST) X-Forwarded-Encrypted: i=1; AJvYcCV4cBX6L9iOJfrm8GfgGub1T4GKYRFIXdcnKso1jmBUBcNRrHcBkjZu8Ko4QRyEaS7NVE+IdQIG5g==@kvack.org X-Received: by 2002:a05:6402:3511:b0:5d3:d7b0:b834 with SMTP id 4fb4d7f45d1cf-5dc5efbdc80mr17093042a12.1.1738451912012; Sat, 01 Feb 2025 15:18:32 -0800 (PST) MIME-Version: 1.0 References: <20250131225837.972218232@goodmis.org> <20250131225942.365475324@goodmis.org> <20250201180810.1faf4906@batman.local.home> In-Reply-To: <20250201180810.1faf4906@batman.local.home> From: Linus Torvalds Date: Sat, 1 Feb 2025 15:18:15 -0800 X-Gmail-Original-Message-ID: X-Gm-Features: AWEUYZmCClD8GCIsriJ_eXSqgwv6IsNbs7qWhVjH3BZVbVBbtFnqEYYSxzE1ZZ4 Message-ID: Subject: Re: [RFC][PATCH 1/2] sched: Extended scheduler time slice To: Steven Rostedt Cc: Mathieu Desnoyers , linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, Thomas Gleixner , Peter Zijlstra , Ankur Arora , linux-mm@kvack.org, x86@kernel.org, akpm@linux-foundation.org, luto@kernel.org, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, willy@infradead.org, mgorman@suse.de, jon.grimm@amd.com, bharata@amd.com, raghavendra.kt@amd.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com, jgross@suse.com, andrew.cooper3@citrix.com, Joel Fernandes , Vineeth Pillai , Suleiman Souhlal , Ingo Molnar , Clark Williams , bigeasy@linutronix.de, daniel.wagner@suse.com, joseph.salisbury@oracle.com, broonie@gmail.com Content-Type: text/plain; charset="UTF-8" X-Rspamd-Queue-Id: 0192D1C000A X-Stat-Signature: ikz9tc997kk37wds567rxymm7zu39ihn X-Rspamd-Server: rspam08 X-Rspam-User: X-HE-Tag: 1738452295-401414 X-HE-Meta: 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 YkYjcmKy ILpr70us3RGATXyCo69RLmPQoBg1ZQXNOFhi6Hb3n/0ThZXt/UC4a5sfLb2uSNLJF//2RiPGH7fKS2k+A/qsww3osyFBpl2ovAQqRrUaj6DpvAkW6AX16LmgM1SSXL3gXWXW+sFVowPpYXXgy+/4DLndrQrvZrQTih8w0+FuQBEr5CD50+k4Hdf+iWY8UQgyY51whQuTygVHNe/FoiuSPluA4EXZNnNrWWLZyPkVbddbIExYYOzJl4NSMyev2kRriWT+hHpzCME2NLwobDYT4oA1H7fMre7bM235RPKki9hF0rulHyhjFsTE11k7wRi/hK+Te4wW0BKO8kyAJSIQSgoT9Rdwm37pfTrq6ugQzaev9Cwof6rFh5lxno0Y1tN7yuMqfk781JsYWw+UoUv5C+9anc9mm8d1bIo+SxOikDjQ4roMNBDJBSO2nky93fYwCzUGja3uehApv7InD9S5SOaqT90Oz7s1zKJkyr6k0DoaEATAacIcT9rriVO8Z630zQXNkAUV95T/YlgDhoQCn66/QLkPGIV+t4dzQqhskojMJgubAmnWloRit/aBc0YECbF1eT9OQz9Q2ai6Q4j29LTysX5rZRLwSebUYrGeSYs//3RuN/fzwO2nV+MpXSYBHztmoNTNeG6+ghZ0= 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 Sat, 1 Feb 2025 at 15:08, Steven Rostedt wrote: > > No it doesn't need to be yield. That just seemed like the obvious > system call to use. But any system call would force a schedule. We > could just optimize yield() though. No, "optimizing" yield is not on the table. Why? Because it has *semantics* that people depend on because it has historical meaning. Things like "move the process to the end of the scheduling queue of that priority". That may sound like a good thing, but it's ABSOLUTELY NOT what you should actually do unless you know *exactly* what the system behavior is. For example, maybe the reason the kernel set NEED_RESCHED_LAZY is that a higher-priority process is ready to run. You haven't used up your time slice yet, but something more important needs the CPU. If you call "sched_yield()", sure, you'll run that higher priority thing. So far so good. But you *also* are literally telling the scheduler to put you at the back of the queue, despite the fact that maybe you are still in line to be run for *your* priority level. So now your performance will absolutely suck, because you just told the scheduler that you are not important, and other processes in your priority level should get priority. So no. "yield()" does not mean "just reschedule". It rally means "yield my position in the scheduling queue". You are literally better off using absolutely *ANY* other system call. The fact that you are confused about this kind of very basic issue does imply that this patch should absolutely be handled by somebody who knows the scheduler better. Linus