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b=ZIJ5FSmcUX5mE5dzwIsn7WJ9SqcBtqdgO52xoBiFN8kEr1AhNI4KxuxdomqHNi3Q4GluuUJRYxd3XAfqpLZO3SkXgWoNhYIO7BOeQWSuiSZjJmfossOkPuQDL4W7ZO4/QQI9gTCXSglGjLfGLIPehC4fuefXpHMsxNrh74TEdnM= Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=amd.com; Received: from BL1PR12MB5320.namprd12.prod.outlook.com (2603:10b6:208:314::17) by DS0PR12MB8342.namprd12.prod.outlook.com (2603:10b6:8:f9::15) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.92.13; Mon, 8 Jun 2026 18:17:52 +0000 Received: from BL1PR12MB5320.namprd12.prod.outlook.com ([fe80::1876:4a6d:2cf5:b8d1]) by BL1PR12MB5320.namprd12.prod.outlook.com ([fe80::1876:4a6d:2cf5:b8d1%5]) with mapi id 15.21.0092.007; Mon, 8 Jun 2026 18:17:52 +0000 Message-ID: <1d7c79bf-1e40-4db7-8f66-45f234b6d87e@amd.com> Date: Mon, 8 Jun 2026 13:17:49 -0500 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 04/12] x86,fs/resctrl: Program PLZA through kmode arch hooks To: Qinyun Tan , "Moger, Babu" Cc: "Luck, Tony" , Reinette Chatre , corbet@lwn.net, Dave.Martin@arm.com, james.morse@arm.com, tglx@kernel.org, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, feng.tang@linux.alibaba.com, linux-kernel@vger.kernel.org References: <0cfd813e10072eefc8f4d84328e83bd9a6220ad4.1777591497.git.babu.moger@amd.com> <1a410ca9-f4a2-4956-8477-033d61a733be@amd.com> <20260605100642.1103628-1-qinyuntan@linux.alibaba.com> Content-Language: en-US From: Babu Moger In-Reply-To: <20260605100642.1103628-1-qinyuntan@linux.alibaba.com> Content-Type: text/plain; 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On 6/5/26 05:06, Qinyun Tan wrote: > On Wed, May 20, 2026 at 06:09:27PM -0500, Moger, Babu wrote: >> On 5/20/2026 5:16 PM, Luck, Tony wrote: >>> So maybe I'm just confused by the name >>> "global_assign_ctrl_inherit_mon_per_cpu" >>> >>> That sounds like "Use the CLOSID from PLZA, but keep the RMID from >>> legacy PQR_ASSOC. >> >>> To make that happen, I thing the PLZA MSR should have rmid_en = 0. >>> But the only code I see that sets this always sets rmid_en=1. >> >> Setting rmid_en = 0 in [1] disables counting of kernel usage for >> RMID 1 (from ctrl1). > > I've been reviewing these patches and went back to the AMD hardware > manual (Publication #69193, Rev 1.00, March 2026) to understand the > RMID_EN semantics. > > The spec defines RMID_EN (bit 31 of PQR_PLZA_ASSOC, MSR C000_03FCh): > > "RMID_EN (PLZA Resource Monitoring Identifier Association Enable) > - bit 31. When set and PLZA_EN is set, the Resource Monitoring > Identifier (RMID) for this logical processor executing at CPL=0 > is specified in the RMID field instead of PQR_ASSOC[RMID]." > > Reading "instead of PQR_ASSOC[RMID]", my understanding is that when > RMID_EN is clear, the hardware falls back to PQR_ASSOC[RMID] - the > user process's RMID - rather than disabling monitoring entirely. > If that reading is correct, RMID_EN would be a source selector > between PLZA and PQR_ASSOC, not an on/off switch for counting. > > Babu, could you confirm whether that is the intended hardware > behavior? Yes. That is correct. RMID_EN is a source selector. > >> The key difference between the two modes is: >> >> In one mode, user and kernel usage are counted together. >> In the other mode, kernel usage is counted separately from user usage. > > If RMID_EN=0 does fall back to PQR_ASSOC[RMID], then the "counted > together" behavior only holds when the user process happens to be in > the same group as the PLZA-bound group. Taking Tony's example: > > # mkdir ctrl1 # CLOSID=1, RMID=1, bound to inherit_mon via PLZA > # mkdir ctrl2 # CLOSID=2, RMID=2 > # echo $$ > ctrl2/tasks > > With rmid_en=1 (current code): when the shell (in ctrl2, PQR_ASSOC > RMID=2) enters kernel mode, kernel activity is counted under PLZA > RMID=1. The user process's RMID is NOT inherited. > > With rmid_en=0: kernel mode would fall back to PQR_ASSOC RMID=2, > correctly inheriting the monitoring identity of the current user > process - which seems to be what "inherit_mon" intends. > > If this reading is right, Tony's suggestion of adding a parameter to > control rmid_en based on the mode would be the right fix: > > void resctrl_arch_configure_kmode(const struct cpumask *cpu_mask, > u32 closid, u32 rmid, > bool assign_rmid, bool enable) > > where assign_rmid=true sets RMID_EN=1 (assign_mon) and > assign_rmid=false sets RMID_EN=0 (inherit_mon). Yes. You are right. I will fix it next revision. > > --- > > Separately, I have a question about the group type constraint for > global_assign_ctrl_assign_mon_per_cpu (assign_mon mode). > > In patch 08/12 ("fs/resctrl: Make info/kernel_mode writable and > identify the bound group"), resctrl_kernel_mode_write() enforces that > assign_mon must bind to an RDTMON_GROUP: > > if (kmode == BIT(GLOBAL_ASSIGN_CTRL_ASSIGN_MON_PER_CPU) && > rdtgrp->type != RDTMON_GROUP) { > > This forces the user to create a ctrl group plus a monitor sub-group. > But since the kernel-mode group is dedicated to kernel work and no > user tasks are assigned to it, the ctrl group's own RMID (allocated > via alloc_rmid() at mkdir time) should be clean and usable. > Requiring an extra monitor group seems to waste a scarce RMID. > > Would it make sense to relax this constraint so that assign_mon also > accepts an RDTCTRL_GROUP? The path infrastructure > (rdtgroup_by_kmode_path and resctrl_kernel_mode_show) already handles > the "//" form, so only the check above would need to change: > > mkdir /sys/fs/resctrl/kernel_ctrl > echo "global_assign_ctrl_assign_mon_per_cpu:group=kernel_ctrl//" \ > > info/kernel_mode > Yes. That is reasonable. assign_mon can be RDTMON_GROUP or RDTCTRL_GROUP. inherit_mon can only be RDTCTRL_GROUP. Thanks Babu