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bh=rhjnF6tfRLQdUd01QFyHkY3TGxnaAW3bBB2Ud2LRuAk=; b=mpcJpAYqKfGfV0JdqA+cD/wJ6gqImAzG8BsS+EJvXXWOMNGDTCgxy+4tH6EHAxvf1H1y09ZIqBELR4CQzPLlBB58CKNswbjhIYD235QIZfEyro2FjaQ6SgMoK/HtmgUu7cEns1rIi86kShturd+0/HTr/JUjq+WlCrgr8HsmCdY= Received: from CO6PR10MB5409.namprd10.prod.outlook.com (2603:10b6:5:357::14) by PH7PR10MB7801.namprd10.prod.outlook.com (2603:10b6:510:308::19) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.7698.29; Tue, 25 Jun 2024 01:17:21 +0000 Received: from CO6PR10MB5409.namprd10.prod.outlook.com ([fe80::25a9:32c2:a7b0:de9e]) by CO6PR10MB5409.namprd10.prod.outlook.com ([fe80::25a9:32c2:a7b0:de9e%3]) with mapi id 15.20.7698.025; Tue, 25 Jun 2024 01:17:21 +0000 References: <20240430183730.561960-1-ankur.a.arora@oracle.com> <20240430183730.561960-9-ankur.a.arora@oracle.com> <20240619121711.jid3enfzak7vykyn@bogus> <871q4pc0f9.fsf@oracle.com> User-agent: mu4e 1.4.10; emacs 27.2 From: Ankur Arora To: Sudeep Holla 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, 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, joao.m.martins@oracle.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com Subject: Re: [PATCH 8/9] arm64: support cpuidle-haltpoll In-reply-to: Date: Mon, 24 Jun 2024 18:17:19 -0700 Message-ID: <87bk3pakio.fsf@oracle.com> Content-Type: text/plain X-ClientProxiedBy: MW4PR03CA0312.namprd03.prod.outlook.com (2603:10b6:303:dd::17) To CO6PR10MB5409.namprd10.prod.outlook.com (2603:10b6:5:357::14) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CO6PR10MB5409:EE_|PH7PR10MB7801:EE_ X-MS-Office365-Filtering-Correlation-Id: 658ee3fc-844e-4f61-b961-08dc94b48ffe X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230037|376011|7416011|366013|1800799021; 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Also select ARCH_HAS_OPTIMIZED_POLL since we have >> >> an optimized polling mechanism via smp_cond_load*(). >> >> >> >> Add the configuration option, ARCH_CPUIDLE_HALTPOLL to allow >> >> cpuidle-haltpoll to be selected. >> >> >> >> Note that we limit cpuidle-haltpoll support to when the event-stream is >> >> available. This is necessary because polling via smp_cond_load_relaxed() >> >> uses WFE to wait for a store which might not happen for an prolonged >> >> period of time. So, ensure the event-stream is around to provide a >> >> terminating condition. >> >> >> > >> > Currently the event stream is configured 10kHz(1 signal per 100uS IIRC). >> > But the information in the cpuidle states for exit latency and residency >> > is set to 0(as per drivers/cpuidle/poll_state.c). Will this not cause any >> > performance issues ? >> >> No I don't think there's any performance issue. >> > > Thanks for the confirmation, that was my assumption as well. > >> When the core is waiting in WFE for &thread_info->flags to >> change, and set_nr_if_polling() happens, the CPU will come out >> of the wait quickly. >> So, the exit latency, residency can be reasonably set to 0. >> > > Sure > >> If, however, there is no store to &thread_info->flags, then the event >> stream is what would cause us to come out of the WFE and check if >> the poll timeout has been exceeded. >> In that case, there was no work to be done, so there was nothing >> to wake up from. >> > > This is exactly what I was referring when I asked about performance, but > it looks like it is not a concern for the reason specified about. > >> So, in either circumstance there's no performance loss. >> >> However, when we are polling under the haltpoll governor, this might >> mean that we spend more time polling than determined based on the >> guest_halt_poll_ns. But, that would only happen in the last polling >> iteration. >> >> So, I'd say, at worst no performance loss. But, we would sometimes >> poll for longer than necessary before exiting to the host. >> > > Does it make sense to add some comment that implies briefly what we > have discussed here ? Mainly why 0 exit and target residency values > are fine and how worst case WFE wakeup doesn't impact the performance. Yeah let me thresh out the commit message for this patch a bit more. Thanks for the review! -- ankur