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bh=SxCb7sJzOgMHli4PuGHDSqcmWdzh5ZnHk0EljJtp7vo=; b=GSeIsijFuJnmXbkUBiKFBQVp1uCwVsLNUHag+xJAUBaVJClPTu9qOyh+Qe3yGWKAlOCYwQiZdbQkGTG38KMQf6OOgH9cA4jjw69TK+tYXs0+tb7eBqmX+fGHVylXqiK4c+/RsqXPyBPxjIi6m/G/CzzcSVQbZIp0DtiJD1JYI5c= Received: from CO6PR10MB5409.namprd10.prod.outlook.com (2603:10b6:5:357::14) by PH8PR10MB6574.namprd10.prod.outlook.com (2603:10b6:510:226::14) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.8137.20; Fri, 8 Nov 2024 22:15:54 +0000 Received: from CO6PR10MB5409.namprd10.prod.outlook.com ([fe80::25a9:32c2:a7b0:de9e]) by CO6PR10MB5409.namprd10.prod.outlook.com ([fe80::25a9:32c2:a7b0:de9e%5]) with mapi id 15.20.8137.019; Fri, 8 Nov 2024 22:15:54 +0000 References: <20241107190818.522639-1-ankur.a.arora@oracle.com> <20241107190818.522639-2-ankur.a.arora@oracle.com> <9cecd8a5-82e5-69ef-502b-45219a45006b@gentwo.org> <87v7wy2mbi.fsf@oracle.com> <88b3b176-97c7-201e-0f89-c77f1802ffd9@gentwo.org> User-agent: mu4e 1.4.10; emacs 27.2 From: Ankur Arora To: "Christoph Lameter (Ampere)" Cc: Ankur Arora , linux-pm@vger.kernel.org, kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-arch@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, 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, maz@kernel.org, misono.tomohiro@fujitsu.com, maobibo@loongson.cn, zhenglifeng1@huawei.com, joao.m.martins@oracle.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com Subject: Re: [PATCH v9 01/15] asm-generic: add barrier smp_cond_load_relaxed_timeout() In-reply-to: <88b3b176-97c7-201e-0f89-c77f1802ffd9@gentwo.org> Date: Fri, 08 Nov 2024 14:15:53 -0800 Message-ID: <87zfm9z812.fsf@oracle.com> Content-Type: text/plain X-ClientProxiedBy: MW4PR04CA0198.namprd04.prod.outlook.com (2603:10b6:303:86::23) To CO6PR10MB5409.namprd10.prod.outlook.com (2603:10b6:5:357::14) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CO6PR10MB5409:EE_|PH8PR10MB6574:EE_ X-MS-Office365-Filtering-Correlation-Id: facddfb2-73cb-4ea8-a241-08dd0042e9b3 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|1800799024|376014|366016; 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And, so you want to minimize >> your power/thermal envelope. > > On ARM that maps to YIELD which does not do anything for the power > envelope AFAICT. It switches to the other hyperthread. Agreed. For arm64 patch-5 adds a specialized version. For the fallback case when we don't have an event stream, the arm64 version does use the same cpu_relax() loop but that's not a production thing. >> For instance see commit 4dc2375c1a4e "cpuidle: poll_state: Avoid >> invoking local_clock() too often" which originally added a similar >> rate limit to poll_idle() where they saw exactly that issue. > > Looping w/o calling local_clock may increase the wait period etc. Yeah. I don't think that's a real problem for the poll_idle() case as the only thing waiting on the other side of the possibly delayed timer is a deeper idle state. But, for any other potential users the looping duration might be too long (the generated code for x86 will execute around 200 * 7 instructions before checking the timer, so a worst case delay of say around 1-2us.) I'll note that in the comment around smp_cond_time_check_count just to warn any future users. > For power saving most arches have special instructions like ARMS > WFE/WFET. These are then causing more accurate wait times than the looping > thing? Definitely true for WFET. The WFE can still overshoot because the eventstream has a period of 100us. -- ankur