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bh=x4AvTb3zlqmSvAt+q7EetleSvRMqPNc3muJW4ddwqr4=; b=JzAOmC8hfwlLx3aWeT7Yt2ySyKTViQLVa1zmOxGK3iRA6QzQllouOchuFhxRAGe7lTRhEPWZBspNQdCp02ixYdviWQqzcqwUvWMgjJ2Q1fA0yZS4cI9rFcn2bmqj/vBBU8bzbtyEUzo+pzUulY3iL9lD+AIgbbw0KjpYzuHoOY4= Received: from CO6PR10MB5409.namprd10.prod.outlook.com (2603:10b6:5:357::14) by CY8PR10MB7196.namprd10.prod.outlook.com (2603:10b6:930:72::14) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.8048.26; Wed, 16 Oct 2024 21:55:11 +0000 Received: from CO6PR10MB5409.namprd10.prod.outlook.com ([fe80::25a9:32c2:a7b0:de9e]) by CO6PR10MB5409.namprd10.prod.outlook.com ([fe80::25a9:32c2:a7b0:de9e%4]) with mapi id 15.20.8069.016; Wed, 16 Oct 2024 21:55:10 +0000 References: <20240925232425.2763385-1-ankur.a.arora@oracle.com> <8634kx5yqd.wl-maz@kernel.org> User-agent: mu4e 1.4.10; emacs 27.2 From: Ankur Arora To: Marc Zyngier Cc: Ankur Arora , linux-pm@vger.kernel.org, kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, catalin.marinas@arm.com, will@kernel.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, pbonzini@redhat.com, wanpengli@tencent.com, vkuznets@redhat.com, rafael@kernel.org, daniel.lezcano@linaro.org, peterz@infradead.org, arnd@arndb.de, lenb@kernel.org, mark.rutland@arm.com, harisokn@amazon.com, mtosatti@redhat.com, sudeep.holla@arm.com, cl@gentwo.org, misono.tomohiro@fujitsu.com, maobibo@loongson.cn, joao.m.martins@oracle.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com Subject: Re: [PATCH v8 00/11] Enable haltpoll on arm64 In-reply-to: <8634kx5yqd.wl-maz@kernel.org> Date: Wed, 16 Oct 2024 14:55:09 -0700 Message-ID: <87plnzpvb6.fsf@oracle.com> Content-Type: text/plain X-ClientProxiedBy: MW3PR05CA0029.namprd05.prod.outlook.com (2603:10b6:303:2b::34) To CO6PR10MB5409.namprd10.prod.outlook.com (2603:10b6:5:357::14) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CO6PR10MB5409:EE_|CY8PR10MB7196:EE_ X-MS-Office365-Filtering-Correlation-Id: 05e7f0ee-4f0a-4a86-f842-08dcee2d34d2 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|366016|1800799024; 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This is specifically interesting for KVM guests by >> reducing IPC latencies. >> >> Comparing idle switching latencies on an arm64 KVM guest with >> perf bench sched pipe: >> >> usecs/op %stdev >> >> no haltpoll (baseline) 13.48 +- 5.19% >> with haltpoll 6.84 +- 22.07% >> >> >> No change in performance for a similar test on x86: >> >> usecs/op %stdev >> >> haltpoll w/ cpu_relax() (baseline) 4.75 +- 1.76% >> haltpoll w/ smp_cond_load_relaxed() 4.78 +- 2.31% >> >> Both sets of tests were on otherwise idle systems with guest VCPUs >> pinned to specific PCPUs. One reason for the higher stdev on arm64 >> is that trapping of the WFE instruction by the host KVM is contingent >> on the number of tasks on the runqueue. > > Sorry to state the obvious, but if that's the variable trapping of > WFI/WFE is the cause of your trouble, why don't you simply turn it off > (see 0b5afe05377d for the details)? Given that you pin your vcpus to > physical CPUs, there is no need for any trapping. Good point. Thanks. That should help reduce the guessing games around the variance in these tests. -- ankur