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Wed, 13 May 2026 07:37:42 -0700 (PDT) Received: from jlelli-thinkpadt14gen4.remote.csb ([151.29.56.132]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48fc8d1986fsm72175295e9.3.2026.05.13.07.37.40 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 13 May 2026 07:37:40 -0700 (PDT) Date: Wed, 13 May 2026 16:37:38 +0200 From: Juri Lelli To: Yuri Andriaccio Cc: Peter Zijlstra , Yuri Andriaccio , Ingo Molnar , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , linux-kernel@vger.kernel.org, Luca Abeni Subject: Re: [RFC PATCH v5 19/29] sched/rt: Remove support for cgroups-v1 Message-ID: References: <20260430213835.62217-1-yurand2000@gmail.com> <20260430213835.62217-20-yurand2000@gmail.com> <20260505150144.GE3102624@noisy.programming.kicks-ass.net> <913f3f99-cd70-43ac-9a9e-c56f5d4c23a6@santannapisa.it> Precedence: bulk X-Mailing-List: linux-kernel@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: <913f3f99-cd70-43ac-9a9e-c56f5d4c23a6@santannapisa.it> On 13/05/26 14:15, Yuri Andriaccio wrote: > Hello, > > > This however means we will essentially need to maintain 2 versions of > > rt.c until v1 is gone? AFAIK v1 rt group implementation is quite a > > substantial amount of code. :/ > > If we really need to maintain cgroups v1 maybe it is possible to use the > same code for HCBS (as I think the behaviors of the old RT_GROUP_SCHED and > HCBS are 'compatible') and use a subset of the functionalities and > configuration modes for cgroup v2, i.e., use the cpu.rt_runtime/period_us > files and disallow running tasks on the root runqueue if they do not belong > in the root cgroup, which is basically what already happens with > RT_GROUP_SCHED (right?). > > So this would mean that cgroups v1 will still be 'broken' but will be > compatible with what there is already. And a good amount of old code will be > gone. > > Does this make sense? Yeah it does, but I'd still be worried about what happens to existing v1 users. Today RT_GROUP_SCHED runs at RT priority and can balance runtime between CPUs, so tasks don't migrate while runtime moves to them. With HCBS we'd be switching to deadline servers running at DEADLINE priority, and tasks migrate to where runtime is available instead. So v1 users would see their cgroup tasks suddenly running above other RT tasks, and the switch from runtime balancing to task migration could mess with cache locality and CPU pinning setups that people have tuned around the current behavior. Keeping the same interface files doesn't really hide that we're changing the whole mechanism underneath. :/ BTW, these concerns apply to v2 as well and we will probably need to discuss them at some point. But, v2 RT support is new, so breaking current setups might be to expect to a certain extent?