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bh=qiETNU/aOnJqbrljgNNriLexfZ7GDOgVZa9miK3MfFg=; b=xBqyYlIm9JMLRDue4ZDhZHP1+ZLc0J6e7amN0hyF6GALm1vOapXXOfXdBfwRDTPT3HyMNJzShyPcBpP4GpcR0gDKPxhmumJOQOSjx/Mtrhw23DSoMi3Sp5CFo2/dnJeJRIS1NzFoz1CcrBdnaj8dxHXyyW5G2dt18Etz1Pb3W0w= Received: from CO6PR10MB5409.namprd10.prod.outlook.com (2603:10b6:5:357::14) by CYXPR10MB7900.namprd10.prod.outlook.com (2603:10b6:930:d7::17) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.8069.28; Tue, 22 Oct 2024 22:01:06 +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 15.20.8069.024; Tue, 22 Oct 2024 22:01:06 +0000 References: <20240925232425.2763385-1-ankur.a.arora@oracle.com> <8634kx5yqd.wl-maz@kernel.org> <87plnzpvb6.fsf@oracle.com> <86sesv3zvf.wl-maz@kernel.org> <874j5apofp.fsf@oracle.com> User-agent: mu4e 1.4.10; emacs 27.2 From: Ankur Arora To: Ankur Arora Cc: Marc Zyngier , 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: <874j5apofp.fsf@oracle.com> Date: Tue, 22 Oct 2024 15:01:03 -0700 Message-ID: <877c9zhk68.fsf@oracle.com> Content-Type: text/plain X-ClientProxiedBy: BYAPR05CA0062.namprd05.prod.outlook.com (2603:10b6:a03:74::39) To CO6PR10MB5409.namprd10.prod.outlook.com (2603:10b6:5:357::14) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CO6PR10MB5409:EE_|CYXPR10MB7900:EE_ X-MS-Office365-Filtering-Correlation-Id: 98c397dd-20a9-405f-eae5-08dcf2e5065e X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|7416014|376014; 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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. >> >> I'd be interested to find out whether there is still some benefit in >> this series once you disable the WFx trapping heuristics. > > The benefit of polling in idle is more than just avoiding the cost of > trapping and re-entering. The other benefit is that remote wakeups > can now be done just by setting need-resched, instead of sending an > IPI, and incurring the cost of handling the interrupt on the receiver > side. > > But let me get you some numbers with that. So, I ran the sched-pipe test with processes on VCPUs 4 and 5 with kvm-arm.wfi_trap_policy=notrap. # perf stat -r 5 --cpu 4,5 -e task-clock,cycles,instructions,sched:sched_wake_idle_without_ipi \ perf bench sched pipe -l 1000000 -c 4 # No haltpoll (and, no TIF_POLLING_NRFLAG): Performance counter stats for 'CPU(s) 4,5' (5 runs): 25,229.57 msec task-clock # 2.000 CPUs utilized ( +- 7.75% ) 45,821,250,284 cycles # 1.816 GHz ( +- 10.07% ) 26,557,496,665 instructions # 0.58 insn per cycle ( +- 0.21% ) 0 sched:sched_wake_idle_without_ipi # 0.000 /sec 12.615 +- 0.977 seconds time elapsed ( +- 7.75% ) # Haltpoll: Performance counter stats for 'CPU(s) 4,5' (5 runs): 15,131.58 msec task-clock # 2.000 CPUs utilized ( +- 10.00% ) 34,158,188,839 cycles # 2.257 GHz ( +- 6.91% ) 20,824,950,916 instructions # 0.61 insn per cycle ( +- 0.09% ) 1,983,822 sched:sched_wake_idle_without_ipi # 131.105 K/sec ( +- 0.78% ) 7.566 +- 0.756 seconds time elapsed ( +- 10.00% ) We get a decent boost just because we are executing ~20% fewer instructions. Not sure how the cpu frequency scaling works in a VM but we also run at a higher frequency. -- ankur