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Shenoy" Cc: Mario Limonciello , Borislav Petkov , Perry Yuan , "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , "Rafael J . Wysocki" , "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , "open list:ACPI" , "open list:CPU FREQUENCY SCALING FRAMEWORK" , changwoo@igalia.com, David Vernet , Tejun Heo References: <20240826211358.2694603-1-superm1@kernel.org> <20240826211358.2694603-9-superm1@kernel.org> <61b96549-2969-4b64-a40d-f91f614ec3ab@amd.com> Content-Language: en-US From: Mario Limonciello In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SN7PR04CA0074.namprd04.prod.outlook.com (2603:10b6:806:121::19) To MN0PR12MB6101.namprd12.prod.outlook.com (2603:10b6:208:3cb::10) Precedence: bulk X-Mailing-List: linux-acpi@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: MN0PR12MB6101:EE_|IA1PR12MB7688:EE_ X-MS-Office365-Filtering-Correlation-Id: 9869d3ac-c279-4711-57bb-08dcc82ac059 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|7416014|376014; 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Shenoy wrote: >> Hello Andrea, >> >> On Wed, Aug 28, 2024 at 08:20:50AM +0200, Andrea Righi wrote: >>> On Wed, Aug 28, 2024 at 10:38:45AM +0530, Gautham R. Shenoy wrote: >>> ... >>>>> I had thought this was a malfunction in the behavior that it reflected the >>>>> current status, not the hardware /capability/. >>>>> >>>>> Which one makes more sense for userspace? In my mind the most likely >>>>> consumer of this information would be something a sched_ext based userspace >>>>> scheduler. They would need to know whether the scheduler was using >>>>> preferred cores; not whether the hardware supported it. >>>> >>>> The commandline parameter currently impacts only the fair sched-class >>>> tasks since the preference information gets used only during >>>> load-balancing. >>>> >>>> IMO, the same should continue with sched-ext, i.e. if the user has >>>> explicitly disabled prefcore support via commandline, the no sched-ext >>>> scheduler should use the preference information to make task placement >>>> decisions. However, I would like to see what the sched-ext folks have >>>> to say. Adding some of them to the Cc list. >>> >>> IMHO it makes more sense to reflect the real state of prefcore support >>> from a "system" perspective, more than a "hardware" perspective, so if >>> it's disabled via boot command line it should show disabled. >>> >>> From a user-space scheduler perspective we should be fine either way, as >>> long as the ABI is clearly documented, since we also have access to >>> /proc/cmdline and we would be able to figure out if the user has >>> disabled it via cmdline (however, the preference is still to report the >>> actual system status). >> >> Thank you for confirming this. >> >>> >>> Question: having prefcore enabled affects also the value of >>> scaling_max_freq? Like an `lscpu -e`, for example, would show a higher >>> max frequency for the specific preferred cores? (this is another useful >>> information from a sched_ext scheduler perspective). >> >> Since the scaling_max_freq is computed based on the boost-numerator, >> at least from this patchset, the numerator would be the same across >> all kinds of cores, and thus the scaling_max_freq reported will be the >> same across all the cores. > > I see, so IIUC from user-space the most reliable way to detect the > fastest cores is to check amd_pstate_highest_perf / amd_pstate_max_freq, > right? I'm trying to figure out a way to abstract and generalize the > concept of "fast cores" in sched_ext. Right now the best way to do this is to look at the amd_pstate_precore_ranking file. In this series there has been some discussion of dropping it though in favor of looking at the highest perf file. I don't believe we're concluded one way or another on it yet though. > > Also, is this something that has changed recently? I see this on an > AMD Ryzen Threadripper PRO 7975WX 32-Cores running a 6.8 kernel: > > $ uname -r > 6.8.0-40-generic You're missing the preferred core patches on this kernel. They landed in 6.9, it's better to upgrade to 6.10.y or 6.11-rc.