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Fri, 8 Nov 2024 07:53:38 +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 07:53:38 +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> 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: <9cecd8a5-82e5-69ef-502b-45219a45006b@gentwo.org> Date: Thu, 07 Nov 2024 23:53:37 -0800 Message-ID: <87v7wy2mbi.fsf@oracle.com> Content-Type: text/plain X-ClientProxiedBy: MW4PR02CA0011.namprd02.prod.outlook.com (2603:10b6:303:16d::28) To CO6PR10MB5409.namprd10.prod.outlook.com (2603:10b6:5:357::14) Precedence: bulk X-Mailing-List: linux-arch@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CO6PR10MB5409:EE_|CH3PR10MB7187:EE_ X-MS-Office365-Filtering-Correlation-Id: 579ffa1a-e5ae-4fcb-91e3-08dcffca7467 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|376014|7416014; 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Can we use cycles and ns instead to have defined periods of > time? Later patches establish the infrastructure to convert cycles to > nanoseconds and microseconds. Use that? > >> +#ifndef smp_cond_load_relaxed_timeout >> +#define smp_cond_load_relaxed_timeout(ptr, cond_expr, time_expr_ns, \ >> + time_limit_ns) ({ \ >> + typeof(ptr) __PTR = (ptr); \ >> + __unqual_scalar_typeof(*ptr) VAL; \ >> + unsigned int __count = 0; \ >> + for (;;) { \ >> + VAL = READ_ONCE(*__PTR); \ >> + if (cond_expr) \ >> + break; \ >> + cpu_relax(); \ >> + if (__count++ < smp_cond_time_check_count) \ >> + continue; \ >> + if ((time_expr_ns) >= time_limit_ns) \ >> + break; \ > > Calling the clock retrieval function repeatedly should be fine and is > typically done in user space as well as in kernel space for functions that > need to wait short time periods. The problem is that you might have multiple CPUs polling in idle for prolonged periods of time. And, so you want to minimize your power/thermal envelope. 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. -- ankur