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Wed, 28 Jan 2026 19:17:03 +0000 Date: Wed, 28 Jan 2026 11:17:01 -0800 From: "Luck, Tony" To: "Moger, Babu" CC: "Moger, Babu" , "corbet@lwn.net" , "reinette.chatre@intel.com" , "Dave.Martin@arm.com" , "james.morse@arm.com" , "tglx@kernel.org" , "mingo@redhat.com" , "bp@alien8.de" , "dave.hansen@linux.intel.com" , "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" , "akpm@linux-foundation.org" , "pawan.kumar.gupta@linux.intel.com" , "pmladek@suse.com" , "feng.tang@linux.alibaba.com" , "kees@kernel.org" , "arnd@arndb.de" , "fvdl@google.com" , "lirongqing@baidu.com" , "bhelgaas@google.com" , "seanjc@google.com" , "xin@zytor.com" , "Shukla, Manali" , "dapeng1.mi@linux.intel.com" , "chang.seok.bae@intel.com" , "Limonciello, Mario" , "naveen@kernel.org" , "elena.reshetova@intel.com" , "Lendacky, Thomas" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "kvm@vger.kernel.org" , "peternewman@google.com" , "eranian@google.com" , "Shenoy, Gautham Ranjal" Subject: Re: RE: [RFC PATCH 13/19] x86/resctrl: Add PLZA state tracking and context switch handling Message-ID: References: <17c9c0c252dcfe707dffe5986e7c98cd121f7cef.1769029977.git.babu.moger@amd.com> <5ec19557-6a62-4158-af82-c70bac75226f@amd.com> Content-Type: text/plain; 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That > > > would avoid any additional context switch overhead as the PLZA MSR would never > > > need to change. > > > > Yes. That can be one use case. Are there other use cases? I think the prime motivation for PLZA is to avoid priority inversions where some task is running with restricted resources, and does a system call, or takes an interrupt, and the kernel is stuck with those limited resources - which may delay context switch to a high priority process. That thinking leads to "run all ring 0 code with more resources". Do you see use cases where you'd like to see the low priority tasks bumped up to some higher level of resources (but perhaps not full access). While medium priority tasks keep same resources when entering ring0. I think there is some slight oddity if a resctrl group that already has some tasks is made into the PLZA group. The existing tasks get marked as PLZA enabled. So run with the same resources in ring0 and ring3. But you can't add a new task to this group with that same property. A newly added task retains its ring3 resources by remaining in its original group. It only gets the PLZA treatment when transitioning to ring0. -Tony