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emacs 27.2 From: Ankur Arora To: Catalin Marinas Cc: "Christoph Lameter (Ampere)" , Ankur Arora , linux-pm@vger.kernel.org, kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, 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, misono.tomohiro@fujitsu.com, maobibo@loongson.cn, joao.m.martins@oracle.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com Subject: Re: [PATCH v8 01/11] cpuidle/poll_state: poll via smp_cond_load_relaxed() In-reply-to: Date: Thu, 17 Oct 2024 12:34:29 -0700 Message-ID: <87r08eo75m.fsf@oracle.com> Content-Type: text/plain X-ClientProxiedBy: MW4PR04CA0117.namprd04.prod.outlook.com (2603:10b6:303:83::32) To CO6PR10MB5409.namprd10.prod.outlook.com (2603:10b6:5:357::14) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CO6PR10MB5409:EE_|DS0PR10MB7126:EE_ X-MS-Office365-Filtering-Correlation-Id: 7aba2c36-18d3-46c2-30b6-08dceee2b85e X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|376014|7416014; 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The above is more like poll every timeout period rather >> > than continuously poll until either the need_resched() condition is true >> > _or_ the timeout expired. From Ankur's email, an IPI may not happen so >> > we don't have any guarantee that WFET will wake up before the timeout. >> > The only way for WFE/WFET to wake up on need_resched() is to use LDXR to >> > arm the exclusive monitor. That's what smp_cond_load_relaxed() does. >> >> Sorry no. The IPI will cause the WFE to continue immediately and not wait >> till the end of the timeout period. > > *If* there is an IPI. The scheduler is not really my area but some > functions like wake_up_idle_cpu() seem to elide the IPI if > TIF_NR_POLLING is set. > > But even if we had an IPI, it still feels like abusing the semantics of > smp_cond_load_relaxed() when relying on it to increment a variable in > the condition check as a result of some unrelated wake-up event. This > API is meant to wait for a condition on a single variable. It cannot > wait on multiple variables and especially not one it updates itself > (even if it happens to work on arm64 under certain conditions). Yeah that makes sense. smp_cond_load_relaxed() uses two separate side-effects to make sure things work: the event-stream and the increment in the conditional. I do want to thresh out smp_cond_load_timeout() a bit more but let me reply to your other mail for that. > My strong preference would be to revive the smp_cond_load_timeout() > proposal from Ankur earlier in the year. Ack that. -- ankur