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"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, linux-arch@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, 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: Message-ID: <87bjyhpcgz.fsf@oracle.com> Date: Thu, 14 Nov 2024 16:28:12 -0800 Content-Type: text/plain X-ClientProxiedBy: MW4PR03CA0198.namprd03.prod.outlook.com (2603:10b6:303:b8::23) 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sfid-20241114_162906_488063_46322930 X-CRM114-Status: GOOD ( 21.18 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Catalin Marinas writes: > On Fri, Nov 08, 2024 at 11:41:08AM -0800, Christoph Lameter (Ampere) wrote: >> On Thu, 7 Nov 2024, Ankur Arora wrote: >> > > 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. >> >> On ARM that maps to YIELD which does not do anything for the power >> envelope AFAICT. It switches to the other hyperthread. > > The issue is not necessarily arm64 but poll_idle() on other > architectures like x86 where, at the end of this series, they still call > cpu_relax() in a loop and check local_clock() every 200 times or so > iterations. So I wouldn't want to revert the improvement in 4dc2375c1a4e > ("cpuidle: poll_state: Avoid invoking local_clock() too often"). > > I agree that the 200 iterations here it's pretty random and it was > something made up for poll_idle() specifically and it could increase the > wait period in other situations (or other architectures). > > OTOH, I'm not sure we want to make this API too complex if the only > user for a while would be poll_idle(). We could add a comment that the > timeout granularity can be pretty coarse and architecture dependent (200 > cpu_relax() calls in one deployment, 100us on arm64 with WFE). Yeah, agreed. Not worth over engineering this interface at least not until there are other users. For now I'll just add a comment mentioning that the time-check is only coarse grained and architecture dependent. -- ankur