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Removed it now. >> -----Original Message----- >> From: Luck, Tony >> Sent: Wednesday, January 28, 2026 11:12 AM >> To: Moger, Babu >> Cc: Babu Moger ; 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; manali.shukla@amd.com; dapeng1.mi@linux.intel.com; >> chang.seok.bae@intel.com; mario.limonciello@amd.com; naveen@kernel.org; >> elena.reshetova@intel.com; thomas.lendacky@amd.com; linux- >> doc@vger.kernel.org; linux-kernel@vger.kernel.org; kvm@vger.kernel.org; >> peternewman@google.com; eranian@google.com; gautham.shenoy@amd.com >> Subject: Re: [RFC PATCH 13/19] x86/resctrl: Add PLZA state tracking and context >> switch handling >> >> On Wed, Jan 28, 2026 at 10:01:39AM -0600, Moger, Babu wrote: >>> Hi Tony, >>> >>> Thanks for the comment. >>> >>> On 1/27/2026 4:30 PM, Luck, Tony wrote: >>>> On Wed, Jan 21, 2026 at 03:12:51PM -0600, Babu Moger wrote: >>>>> @@ -138,6 +143,20 @@ static inline void __resctrl_sched_in(struct >> task_struct *tsk) >>>>> state->cur_rmid = rmid; >>>>> wrmsr(MSR_IA32_PQR_ASSOC, rmid, closid); >>>>> } >>>>> + >>>>> + if (static_branch_likely(&rdt_plza_enable_key)) { >>>>> + tmp = READ_ONCE(tsk->plza); >>>>> + if (tmp) >>>>> + plza = tmp; >>>>> + >>>>> + if (plza != state->cur_plza) { >>>>> + state->cur_plza = plza; >>>>> + wrmsr(MSR_IA32_PQR_PLZA_ASSOC, >>>>> + RMID_EN | state->plza_rmid, >>>>> + (plza ? PLZA_EN : 0) | CLOSID_EN | state- >>> plza_closid); >>>>> + } >>>>> + } >>>>> + >>>> >>>> Babu, >>>> >>>> This addition to the context switch code surprised me. After your >>>> talk at LPC I had imagined that PLZA would be a single global >>>> setting so that every syscall/page-fault/interrupt would run with a >>>> different CLOSID (presumably one configured with more cache and memory >> bandwidth). >>>> >>>> But this patch series looks like things are more flexible with the >>>> ability to set different values (of RMID as well as CLOSID) per group. >>> >>> Yes. this similar what we have with MSR_IA32_PQR_ASSOC. The >>> association can be done either thru CPUs (just one MSR write) or task >>> based association(more MSR write as task moves around). >>>> >>>> It looks like it is possible to have some resctrl group with very >>>> limited resources just bump up a bit when in ring0, while other >>>> groups may get some different amount. >>>> >>>> The additions for plza to the Documentation aren't helping me >>>> understand how users will apply this. >>>> >>>> Do you have some more examples? >>> >>> Group creation is similar to what we have currently. >>> >>> 1. create a regular group and setup the limits. >>> # mkdir /sys/fs/resctrl/group >>> >>> 2. Assign tasks or CPUs. >>> # echo 1234 > /sys/fs/resctrl/group/tasks >>> >>> This is a regular group. >>> >>> 3. Now you figured that you need to change things in CPL0 for this task. >>> >>> 4. Now create a PLZA group now and tweek the limits, >>> >>> # mkdir /sys/fs/resctrl/group1 >>> >>> # echo 1 > /sys/fs/resctrl/group1/plza >>> >>> # echo "MB:0=100" > /sys/fs/resctrl/group1/schemata >>> >>> 5. Assign the same task to the plza group. >>> >>> # echo 1234 > /sys/fs/resctrl/group1/tasks >>> >>> >>> Now the task 1234 will be using the limits from group1 when running in CPL0. >>> >>> I will add few more details in my next revision. >>> >> >> Babu, >> >> I've read a bit more of the code now and I think I understand more. >> >> Some useful additions to your explanation. >> >> 1) Only one CTRL group can be marked as PLZA > > Yes. Correct. > >> 2) It can't be the root/default group > > This is something I added to keep the default group in a un-disturbed, > >> 3) It can't have sub monitor groups >> 4) It can't be pseudo-locked > > Yes. > >> >> Would a potential use case involve putting *all* tasks into the PLZA group? That >> would avoid any additional context switch overhead as the PLZA MSR would never >> need to change. > > Yes. That can be one use case. > >> >> If that is the case, maybe for the PLZA group we should allow user to >> do: >> >> # echo '*' > tasks > > Yea. It can be done. > > Thanks > Babu > >