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Wed, 23 Apr 2025 03:37:11 +0000 Message-ID: <2e118d15-ace5-4a07-a54a-18c9da3d3430@intel.com> Date: Tue, 22 Apr 2025 20:37:11 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 10/26] fs/resctrl: Improve handling for events that can be read from any CPU To: "Luck, Tony" CC: Fenghua Yu , "Wieczor-Retman, Maciej" , Peter Newman , James Morse , Babu Moger , "Drew Fustini" , Dave Martin , "Keshavamurthy, Anil S" , "linux-kernel@vger.kernel.org" , "patches@lists.linux.dev" References: <20250407234032.241215-1-tony.luck@intel.com> <20250407234032.241215-11-tony.luck@intel.com> Content-Language: en-US From: Reinette Chatre In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit X-ClientProxiedBy: MW4PR04CA0052.namprd04.prod.outlook.com (2603:10b6:303:6a::27) To SJ2PR11MB7573.namprd11.prod.outlook.com (2603:10b6:a03:4d2::10) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SJ2PR11MB7573:EE_|CH3PR11MB7763:EE_ X-MS-Office365-Filtering-Correlation-Id: 11d01aa7-936f-430e-885a-08dd821821f7 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|1800799024|366016; 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I >> understood that this is being experimented with [2] and last I heard it >> looks promising. >> >>> >>> The blocking problem is that ARM may not be able to read a counter >>> on a tick_nohz CPU because it may need to sleep. >>> >>> Do we need more options for events: >>> >>> 1) Must be read on a CPU in the right domain // Legacy >>> 2) Can be read from any CPU // My addtion >>> 3) Must be read on a "housekeeping" CPU // James' code in upstream >>> 4) Cannot be read on a tick_nohz CPU // Could be combined with 1 or 2? >> >> I do not see needing additional complexity here. I think it will be simpler >> to just replace use of cpumask_any_housekeeping() in mon_event_read() with >> open code that supports the particular usage. As I understand it is prohibited >> for all CPUs to be in tick_nohz_full_mask so it looks to me as though the >> existing "if (tick_nohz_full_cpu(cpu))" should never be true (since no CPU is being excluded). >> Also, since mon_event_read() has no need to exclude CPUs, just a cpumask_andnot() >> should suffice to determine what remains of given mask after accounting for all the >> NO_HZ CPUs if tick_nohz_full_enabled(). > > Maybe there isn’t much complexity to make this "read one counter" better on systems > where reading from any CPU is possible. Taking your advice from the earlier review > the filesystem code can set a flag in the mon_evt structure. struct mon_data and > struct rmid_read can change from holding the event id to holding a pointer to the > mon_evt (as the source of truth). > > Then mon_event_read() can just have a simple direct call to mon_event_count() > just before the call to cpumask_any_housekeeping() like this: > > if (evt->any_cpu) { > mon_event_count(rr); > goto done; > } > > The "goto done" jumps to the resctrl_arch_mon_ctx_free(r, evt->evtid, rr->arch_mon_ctx); > at the end of mon_event_read() Thanks, this looks great. > > Folks can still pursue the bulk read of many counters (though I expect you might want > one file per domain, rather than a single file to report everything). I will have to re-read that thread to refresh myself on the discussion but scanning the thread I did find a summary of points (end of [3]) and this was there. > >> >> Reinette >> >>> >>>> Reinette >>> >>> [1] https://lore.kernel.org/all/20241031142553.3963058-2-peternewman@google.com/ >>>> >> >> [2] https://lore.kernel.org/lkml/CALPaoCgpnVORZfbKVLXDFUZvv8jhpShHPzB3cwdLTZQH1o9ULw@mail.gmail.com/ > [3] https://lore.kernel.org/lkml/34fd8713-3430-4e27-a2c2-fd8839f90f5a@intel.com/ Reinette