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Fri, 4 Jul 2025 05:57:53 +0000 Message-ID: Date: Fri, 4 Jul 2025 13:57:42 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] sched/numa: Fix NULL pointer access to mm_struct durng task swap To: Peter Zijlstra CC: Michal Hocko , Ingo Molnar , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , Andrew Morton , Tim Chen , , Jirka Hladky , "Srikanth Aithal" , Suneeth D , Libo Chen References: <20250702163247.324439-1-yu.c.chen@intel.com> <20250703072608.GS1613200@noisy.programming.kicks-ass.net> <20250703115006.GT1613200@noisy.programming.kicks-ass.net> <20250703123626.GW1613200@noisy.programming.kicks-ass.net> <20250703140109.GW1613376@noisy.programming.kicks-ass.net> Content-Language: en-US From: "Chen, Yu C" In-Reply-To: <20250703140109.GW1613376@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SG2P153CA0017.APCP153.PROD.OUTLOOK.COM (2603:1096::27) 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_|MN6PR11MB8169:EE_ X-MS-Office365-Filtering-Correlation-Id: 4f8167ea-139c-49fa-d200-08ddbabfb73c X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|7416014|376014|366016; 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The other parts, schedstat and vmstat, are still quite helpful. >>>> Also tracepoints are more expensive than counters once enabled, I think >>>> that's too much for just counting numbers. >>> >>> I'm not generally a fan of eBPF, but supposedly it is really good for >>> stuff like this. >>> >>> Attaching to a tracepoint and distributing into cgroup buckets seems >>> like it should be a trivial script. >> >> Yes, it is feasible to use eBPF. On the other hand, if some >> existing monitoring programs rely on /proc/{pid}/sched to observe >> the NUMA balancing metrics of processes, it might be helpful to >> include the NUMA migration/swap information in /proc/{pid}/sched. >> This approach can minimize the modifications needed for these >> monitoring programs, eliminating the need to add a new BPF script >> to obtain NUMA balancing statistics from different sources IMHO. > > Maybe... > > The thing is, most of the time the effort spend on collecting all these > numbers is wasted energy since nobody ever looks at them. > As for per-task NUMA balancing activity itself, we found it useful for debugging when trying to ensure that cache-aware load balancing coexists properly with NUMA balancing. > Sometimes we're stuck with ABI, like the proc files you mentioned. We > can't readily remove them, stuff would break. But does that mean we > should endlessly add to them just because convenient? > > Ideally I would strip out all the statistics and accounting crap and> make sure we have tracepoints (not trace-events) covering all the needed > spots, and then maybe just maybe have a few kernel modules that hook > into those tracepoints to provide the legacy interfaces. > > That way, only the people that care get to pay the overhead of actually > collecting the numbers. > > One can dream I suppose... :-) I see. If I understand correctly, it's generally not recommended to add new items under /proc. Users are recommended to use tracepoints/events when trying to collect the statistics, and something like schedstat_inc() should be avoided. Is the per-task data an exception? We recently exposed task's slice via /proc/pid/sched : D thanks, Chenyu