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[209.85.208.46]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-5dc724043b7sm4978376a12.40.2025.02.01.15.36.09 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 01 Feb 2025 15:36:10 -0800 (PST) Received: by mail-ed1-f46.google.com with SMTP id 4fb4d7f45d1cf-5d3cf094768so5805107a12.0 for ; Sat, 01 Feb 2025 15:36:09 -0800 (PST) X-Forwarded-Encrypted: i=1; AJvYcCVHfIpEtlJ1gTaKEJGJhoIIFCw62agBkQGNSHBtmOnb4bW3LT9wnnywyPlgTkd4MoX9X/FiSJEpjQ==@kvack.org X-Received: by 2002:a05:6402:909:b0:5d9:f9b8:e7fb with SMTP id 4fb4d7f45d1cf-5dc5efe74cdmr17656373a12.22.1738452969323; Sat, 01 Feb 2025 15:36:09 -0800 (PST) MIME-Version: 1.0 References: <20250131225837.972218232@goodmis.org> <20250131225942.365475324@goodmis.org> <20250201180810.1faf4906@batman.local.home> In-Reply-To: From: Linus Torvalds Date: Sat, 1 Feb 2025 15:35:53 -0800 X-Gmail-Original-Message-ID: X-Gm-Features: AWEUYZnL7HWDqqscez1fIJLpsSsGzS2Qj9VepL3JpazWidbVl6o5BaS06DFvaNM 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-Stat-Signature: 6rrmor9ytxrxx9341q8kpinzid1dwnsb X-Rspam-User: X-Rspamd-Queue-Id: C9DBD100003 X-Rspamd-Server: rspam03 X-HE-Tag: 1738452973-371663 X-HE-Meta: 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 1yZ26yoE 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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 Sat, 1 Feb 2025 at 15:18, Linus Torvalds wrote: > > 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. Side note: we've messed with this before, exactly because people have been confused about this before. I have this dim memory of us having at one point decided to actively ignore that "put me last" part of sched_yield because we had huge performance problems with people misusing "sched_yield()". I didn't actually check what current active schedulers do. I would not be in the least surprised if different schedulers end up having very different behavior (particularly any RT scheduler). But the moral of the story ends up being the same: don't use yield() unless you are on some embedded platform and know exactly what the scheduling pattern is - or if you really want to say "I don't want to run now, do *anything* else, my performance doesn't matter". A traditional (reasonable) situation might be "I started another task or thread, I need for it to run first and initialized things, I'm polling for that to be done but I don't want to busy-loop if there is real work to be done". Where it really is a complete hack: "my performance doesn't matter because it's a one-time startup thing, and I couldn't be arsed to have real locking". In fact, the whole "I couldn't be arsed" is basically the tag-line for "yield()". Maybe we should rename the system call. Linus