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Mon, 13 Apr 2026 08:38:44 +0000 Message-ID: Date: Mon, 13 Apr 2026 16:38:24 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] sched/fair: dynamically scale the period of cache work To: Jianyong Wu CC: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , References: <4fb7a6da-447d-452a-a920-7cd39b939ccb@intel.com> <20260413072309.2663668-1-wujianyong@hygon.cn> Content-Language: en-US From: "Chen, Yu C" In-Reply-To: <20260413072309.2663668-1-wujianyong@hygon.cn> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SI2PR04CA0016.apcprd04.prod.outlook.com (2603:1096:4:197::7) To DM4PR11MB6020.namprd11.prod.outlook.com (2603:10b6:8:61::19) 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: DM4PR11MB6020:EE_|IA3PR11MB9400:EE_ X-MS-Office365-Filtering-Correlation-Id: f92edb6a-7b51-48da-6575-08de99381265 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|7416014|376014|18002099003|56012099003|22082099003; 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Because it scans all system CPUs for > computation, high-frequency execution hurts performance. We thus reduce > the scan rate in such cases. > > On the other hand, if the preferred node becomes suboptimal, we should > increase the scan frequency to quickly find a better placement. The scan > period is therefore dynamically adjusted. > > Signed-off-by: Jianyong Wu > > --- > Hi ChenYu, Tim, Gengkun, > > I have another approach to address this issue, based on the observation > that the scan work can be canceled if the preferred node is stable.This > patch merely demonstrates the idea, but still needs more testing to > verify its functionality. I'm sending it out early to gather feedback and > opinions. > Thanks for providing this patch. > if (work->next == work) { > @@ -1728,7 +1734,7 @@ static void task_cache_work(struct callback_head *work) > struct task_struct *p = current, *cur; > unsigned long curr_m_a_occ = 0; > struct mm_struct *mm = p->mm; > - unsigned long m_a_occ = 0; > + unsigned long m_a_occ = 0, need_scan = 0, now; > cpumask_var_t cpus; > u64 t0, scan_cost; > > @@ -1753,6 +1759,12 @@ static void task_cache_work(struct callback_head *work) > > t0 = sched_clock_cpu(curr_cpu); > > + now = jiffies; > + if (time_before(now, READ_ONCE(mm->sc_stat.next_scan))) > + return; > + I agree that limiting the scan rate would be useful, and your above change is actually similar to what NUMA balancing did in task_numa_work(). It allows only one thread within the same process to perform the statistics calculation, which avoids redundant computation. > + WRITE_ONCE(mm->sc_stat.next_scan, (now + mm->sc_stat.scan_period)); > + I suppose the above should be try_cmpxchg()? That is to say, with your above change, we have already limited the scan ratio for multi-threaded processes significantly. There appears to be no need to perform adaptive adjustment of scan_period - the benefit of introducing an adaptive scan_period may not offset the overhead of frequent writing to the "global" mm->sc_stat.scan_period due to c2c? thanks, Chenyu