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b=L6abealG0h+EtVvDaxuW19bLJ53g4MNd6xAaagGpRvSIZ0zGKocZDxHvQjAeGRQ/5QDvBTWt4EW9b7/l8Krwfo7wsVHY1UYRLF99yfuedENgiUR4W5vLnp2XJQISmBBb6l6cs9ke9gYY3bxvV/W/f0eiLQy4ER+jlhb9kzI3CXA= Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=amd.com; Received: from IA0PPF9A76BB3A6.namprd12.prod.outlook.com (2603:10b6:20f:fc04::bdc) by IA1PR12MB6090.namprd12.prod.outlook.com (2603:10b6:208:3ee::19) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9564.8; Thu, 29 Jan 2026 19:31:30 +0000 Received: from IA0PPF9A76BB3A6.namprd12.prod.outlook.com ([fe80::4d0e:603a:42fc:7c0]) by IA0PPF9A76BB3A6.namprd12.prod.outlook.com ([fe80::4d0e:603a:42fc:7c0%3]) with mapi id 15.20.9542.015; Thu, 29 Jan 2026 19:31:29 +0000 Message-ID: <53abadfa-a429-401d-b9c8-71149b0b960d@amd.com> Date: Thu, 29 Jan 2026 13:31:26 -0600 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 16/19] fs/resctrl: Implement rdtgroup_plza_write() to configure PLZA in a group To: "Luck, Tony" , Babu Moger Cc: 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 References: Content-Language: en-US From: Babu Moger In-Reply-To: Content-Type: text/plain; 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Also, ensure that only one group can have PLZA >>> enabled at a time. >>> >> ... >> >>> +static ssize_t rdtgroup_plza_write(struct kernfs_open_file *of, char *buf, >>> + size_t nbytes, loff_t off) >>> +{ >>> + struct rdt_resource *r = resctrl_arch_get_resource(RDT_RESOURCE_L3); >>> + struct rdtgroup *rdtgrp, *prgrp; >>> + int cpu, ret = 0; >>> + bool enable; >> ... >> >>> + /* Enable or disable PLZA state and update per CPU state if there is a change */ >>> + if (enable != rdtgrp->plza) { >>> + resctrl_arch_plza_setup(r, rdtgrp->closid, rdtgrp->mon.rmid); >> What is this for? If I've just created a group with no tasks, and empty >> CPU mask ... it seems that this writes the MSR_IA32_PQR_PLZA_ASSOC on >> every CPU in every domain. Here is the reason. Some fields of PQR_PLZA_ASSOC must be set to the same value for all HW threads in the QOS domain for consistent operation (Per-QosDomain).  The user should use the following sequence to set these values to a consistent state. 1. Set PQR_PLZA_ASSOC[PLZA_EN]=0 for all HW threads in the QOS Domain 2. Set the COS_EN, COS, RMID_EN, and RMID fields of PQR_PLZA_ASSOC to the desired configuration on all HW threads in the QOS Domain 3. Set PQR_PLZA_ASSOC[PLZA_EN]=1 for all HW threads in the QOS Domain where PLZA should be enabled. * The user should perform this as a read-modify-write to avoid changing the value of COS_EN, COS, RMID_EN, and RMID fields of PQR_PLZA_ASSOC. Basically, we have to set all the fields to consistent state to setup the PLZA first.   Then setup PLZA_EN bit on each thread based on current association. > I think I see now. There are THREE enable bits in your > MSR_IA32_PQR_PLZA_ASSOC. > One each for CLOSID and RMID, and an overall PLZA_EN in the high bit. > > At this step you setup the CLOSID/RMID with their enable bits, but > leaving the PLZA_EN off. > > Is this a subtle optimzation for the context switch? Is the WRMSR > faster if it only toggle PLZA_EN leaving all the other bits unchanged? I really did not think of optimization here. Mostly followed the spec. > > This might not be working as expected. The context switch code does: > > wrmsr(MSR_IA32_PQR_PLZA_ASSOC, > RMID_EN | state->plza_rmid, > (plza ? PLZA_EN : 0) | CLOSID_EN | state->plza_closid); > > This doesn't just clear the PLZA_EN bit, it zeroes the high dword of the MSR. > >> It also appears that marking a task as PLZA is permanent. Moving it to >> another group doesn't unmark it. Is this intentional? > Ditto assigning a CPU to the PLZA group. Once done it can't be undone > (except by turing off PLZA?). > > -Tony > > [More comments about this coming against patch 16] >