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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 16 Oct 2025 11:38:23.1632 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: e2bdb1ba-8c80-4950-59b6-08de0ca88375 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: DB1PEPF00050A00.eurprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AM9PR08MB5924 On 10/15/25 11:35, Juri Lelli wrote: > On 14/10/25 21:09, Pingfan Liu wrote: >> Hi Pierre, >> >> Thanks for sharing your perspective. >> >> On Sat, Oct 11, 2025 at 12:26 AM Pierre Gondois wrote: >>> >>> On 10/6/25 14:12, Juri Lelli wrote: >>>> On 06/10/25 12:13, Pierre Gondois wrote: >>>>> On 9/30/25 11:04, Peter Zijlstra wrote: >>>>>> On Tue, Sep 30, 2025 at 08:20:06AM +0100, Juri Lelli wrote: >>>>>> >>>>>>> I actually wonder if we shouldn't make cppc_fie a "special" DEADLINE >>>>>>> tasks (like schedutil [1]). IIUC that is how it is thought to behave >>>>>>> already [2], but, since it's missing the SCHED_FLAG_SUGOV flag(/hack), >>>>>>> it is not "transparent" from a bandwidth tracking point of view. >>>>>>> >>>>>>> 1 -https://elixir.bootlin.com/linux/v6.17/source/kernel/sched/cpufreq_schedutil.c#L661 >>>>>>> 2 -https://elixir.bootlin.com/linux/v6.17/source/drivers/cpufreq/cppc_cpufreq.c#L198 >>>>>> Right, I remember that hack. Bit sad its spreading, but this CPPC thing >>>>>> is very much like the schedutil one, so might as well do that I suppose. >>>>> IIUC, the sugov thread was switched to deadline to allow frequency updates >>>>> when deadline tasks start to run. I.e. there should be no point updating the >>>>> freq. after the deadline task finished running, cf [1] and [2] >>>>> >>>>> The CPPC FIE worker should not require to run that quickly as it seems to be >>>>> more like a freq. maintenance work (the call comes from the sched tick) >>>>> >>>>> sched_tick() >>>>> \-arch_scale_freq_tick() / topology_scale_freq_tick() >>>>> \-set_freq_scale() / cppc_scale_freq_tick() >>>>> \-irq_work_queue() >>>> OK, but how much bandwidth is enough for it (on different platforms)? >>>> Also, I am not sure the worker follows cpusets/root domain changes. >>>> >>>> >>> To share some additional information, I could to reproduce the issue by >>> creating as many deadline tasks with a huge bandwidth that the platform >>> allows it: >>> chrt -d -T 1000000 -P 1000000 0 yes > /dev/null & >>> >>> Then kexec to another kernel. The available bandwidth of the root domain >>> gradually decreases with the number of CPUs unplugged. >>> At some point, there is not enough bandwidth and an overflow is detected. >>> (Same call stack as in the original message). > I seem to agree with Pingfan below, kexec (kernel crash?) is a case > where all guarantees are out of the window anyway, so really no point in > keeping track of bandwidth and failing hotplug. Guess we should be > adding an ad-hoc check/bail for this case. Yes right >>> So I'm not sure this is really related to the cppc_fie thread. >>> I think it's more related to checking the available bandwidth in a context >>> which is not appropriate. The deadline bandwidth might lack when the >>> platform >>> is reset, but this should not be that important. >>> >> I think there are two independent issues. >> >> In your experiment, as CPUs are hot-removed one by one, at some point >> the hot-removal will fail due to insufficient DL bandwidth. There >> should be a warning message to inform users about what's happening, >> and users can then remove some DL tasks to continue the CPU >> hot-removal. >> >> Meanwhile, in the kexec case, this checking can be skipped since the >> system cannot roll back to a working state anyway Yes right, I meant that: - when using kexec, the kernel crashes - when manually unplugging CPUs with: `echo 0 > /sys/devices/system/cpu/cpuX/online` The kernel returns `write error: Device or resource busy` at some point to prevent from reducing the DL bandwidth too much. ------ I  could not reproduce the issue you reported initially. I am using a radxa orion o6 which has a cppc_fie worker. AFAIU it should not be possible to add/remove bandwidth to the def_root_domain. During kexec, the following is happening to all CPUs: \-dl_bw_manage(dl_bw_req_deactivate, cpu)   \- // Check if there is enough bandwidth \-dl_clear_root_domain_cpu(cpu)   \- // Recompute the available bandwidth based on the remaining CPUs So I'm not sure to understand why accounting some bandwidth to the def_root_domain is problematic in practice as the def_root_domain seems to have some DL bandwidth. IIUC the problem seems to be that for some reason there is not enough bandwidth in the def_root_domain aswell, which triggers the bandwidth overflow detection. >> >> Thanks, >> >> Pingfan >>> --- >>> >>> Question: >>> Since the cppc_fie worker doesn't have the SCHED_FLAG_SUGOV flag, >>> is this comment actually correct ? >>> /* >>> * Fake (unused) bandwidth; workaround to "fix" >>> * priority inheritance. >>> */ >>> >>> --- >>> >>> On a non-deadline related topic, the CPPC drivers creates a cppc_fie >>> worker in >>> case the CPPC counters to estimate the current frequency are in PCC >>> channels. >>> Accessing these channels requires to go through sleeping sections, >>> that's why a worker is used. >>> >>> However, CPPC counters might be accessed through FFH, which doesn't go >>> through >>> sleeping sections. In such case, the cppc_fie worker is never used and never >>> removed, so it would be nice to remote it. >>> >