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[141.239.159.203]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-1f9eb5e0a3esm66355225ad.215.2024.06.24.12.06.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 24 Jun 2024 12:06:39 -0700 (PDT) Sender: Tejun Heo Date: Mon, 24 Jun 2024 09:06:38 -1000 From: Tejun Heo To: Peter Zijlstra Cc: torvalds@linux-foundation.org, mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, martin.lau@kernel.org, joshdon@google.com, brho@google.com, pjt@google.com, derkling@google.com, haoluo@google.com, dvernet@meta.com, dschatzberg@meta.com, dskarlat@cs.cmu.edu, riel@surriel.com, changwoo@igalia.com, himadrics@inria.fr, memxor@gmail.com, andrea.righi@canonical.com, joel@joelfernandes.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org, kernel-team@meta.com Subject: Re: [PATCH 18/39] sched_ext: Allow BPF schedulers to disallow specific tasks from joining SCHED_EXT Message-ID: References: <20240501151312.635565-1-tj@kernel.org> <20240501151312.635565-19-tj@kernel.org> <20240624124053.GN31592@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240624124053.GN31592@noisy.programming.kicks-ass.net> Hello, Peter. On Mon, Jun 24, 2024 at 02:40:53PM +0200, Peter Zijlstra wrote: > On Wed, May 01, 2024 at 05:09:53AM -1000, Tejun Heo wrote: > > BPF schedulers might not want to schedule certain tasks - e.g. kernel > > threads. This patch adds p->scx.disallow which can be set by BPF schedulers > > in such cases. The field can be changed anytime and setting it in > > ops.prep_enable() guarantees that the task can never be scheduled by > > sched_ext. > > Why ?!?! > > By leaving kernel threads fair, and fair sitting above the BPF thing, > it is not dissimilar to promoting them to FIFO. They will instantly > preempt the BPF thing and keep running for as long as they need. The > only real difference between this and actual FIFO is the behaviour on > contention. Yes, from sched_ext's POV, in partial mode, CFS isn't all that different from FIFO. Whenever there are tasks to run in CFS, CPUs are taken away. Right now, partial mode can be useful for leaving a part of system on CFS (e.g. in a cpuset partitioned system), when the scheduler is narrowly focused and doesn't cover everything necessary (e.g. EAS). > This seems like a very bad thing to have, and your 'changelog' has no > justification what so ever. This is a bit of duplicate interface in that in partial mode sched_ext can already be opted in by setting per-thread sched class. However, some use cases wanted this so that the BPF scheduler has the final say over who can be on it rather than the userspace. It's a convenience feature for some use cases. Thanks. -- tejun