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Tue, 21 Apr 2026 16:12:11 +0000 Message-ID: <4fa916b5-1bd0-418e-8d6e-cd1f9a1c8c22@intel.com> Date: Tue, 21 Apr 2026 09:12:07 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 00/16] fs,x86/resctrl: Add kernel-mode (e.g., PLZA) support to the resctrl subsystem To: "Luck, Tony" CC: "Moger, Babu" , Babu Moger , "corbet@lwn.net" , "Dave.Martin@arm.com" , "james.morse@arm.com" , "tglx@kernel.org" , "mingo@redhat.com" , "bp@alien8.de" , "dave.hansen@linux.intel.com" , "skhan@linuxfoundation.org" , "x86@kernel.org" , "hpa@zytor.com" , "peterz@infradead.org" , "juri.lelli@redhat.com" , "vincent.guittot@linaro.org" , "dietmar.eggemann@arm.com" , "rostedt@goodmis.org" , "bsegall@google.com" , "mgorman@suse.de" , "vschneid@redhat.com" , "kas@kernel.org" , "rick.p.edgecombe@intel.com" , "akpm@linux-foundation.org" , "pmladek@suse.com" , "rdunlap@infradead.org" , "dapeng1.mi@linux.intel.com" , "kees@kernel.org" , "elver@google.com" , "paulmck@kernel.org" , "lirongqing@baidu.com" , "safinaskar@gmail.com" , "fvdl@google.com" , "seanjc@google.com" , "pawan.kumar.gupta@linux.intel.com" , "xin@zytor.com" , "tiala@microsoft.com" , "chang.seok.bae@intel.com" , "Lendacky, Thomas" , "elena.reshetova@intel.com" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-coco@lists.linux.dev" , "kvm@vger.kernel.org" , "eranian@google.com" , "peternewman@google.com" References: <5a273b0f-8225-4e9e-924e-884183734659@intel.com> <73c46024-4cf2-4f03-9268-d4378825fa87@amd.com> <973067bf-6e6c-446a-a81a-713840d701a9@intel.com> <90f4a692-1c27-4967-bf12-ec3cb597681d@amd.com> <68a551ea-d9f0-436a-9bef-e35fd027bb95@intel.com> <39e0c786-cc35-4555-bfb9-ff7cd758c423@amd.com> <99a2da36-6a21-4a99-98e0-3c9a4cf7ecf6@amd.com> <58b8fe0c-80f6-4ba2-abbe-90d0ceee6daa@intel.com> Content-Language: en-US From: Reinette Chatre In-Reply-To: Content-Type: text/plain; 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However, there is no >>>> way to know which control group the user intends to assign to kernel mode. >>>> All we know here is the selected mode. >>>> >>>> After this operation, the info/kernel_mode_assignment interface should >>>> become visible. But the question is: what should it contain or point to at >>>> this moment? >>>> >>>> # cat info/kernel_mode_assignment >>>> ?? >>>> >>>> Next operation: Assign the group >>>> >>>> # echo "ctrl1//" > info/kernel_mode_assignment >>> >>> Now ring0 code is using the CLOSID from the ctrl1 group. >> >> ... and user space tasks also continue to use the CLOSID from the >> ctrl1 group. >> It is up to user space to decide if a group is dedicated to kernel >> mode or not. resctrl does not enforce it. >> >>> >>> But the RMID for this group isn't used. >> >> RMID is still used by user mode that maintains existing behavior concerning >> this group when considering its tasks/cpus/cpus_list files. RMID assigned to this >> group is just not used for kernel mode. > > True, that the RMID is used if the user makes assignments using tasks/cpus/cpus_list > for the ctrl1 group. But they might not do that. > >> >>> >>> Are we OK with "wasting" an RMID in this way? >> >> How do you see this RMID as "wasted"? > > Suppose the user doesn't assign tasks to the ctrl1 group? > > Perhaps the resources they want to make available to the kernel do > not exactly match with resources that they want to provide to any > tasks. In this case the RMID is wasted. Under these circumstances, yes, the RMID will not be used. A related scenario (when considering what may happen if user does not assign tasks to the ctrl1 group) is when user space disables PLZA on all CPUs in a domain then the CLOSID (as well as RMID since this is irrespective of rmid_en mode) associated with kernel_mode will be unused in that domain. Reinette