On Wed, 2022-10-12 at 18:58 +0200, Rafael J. Wysocki wrote: > On Tue, Oct 11, 2022 at 3:23 PM Chen Yu <yu.c.chen@xxxxxxxxx> wrote: > > > > Hi Pavel, > > On 2022-10-11 at 13:36:46 +0200, Pavel Machek wrote: > > > Hi! > > > > > > > From: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx> > > > > > > > > [ Upstream commit 68b99e94a4a2db6ba9b31fe0485e057b9354a640 ] > > > > > > > > When CPU 0 is offline and intel_powerclamp is used to inject > > > > idle, it generates kernel BUG: > > > > > > > > BUG: using smp_processor_id() in preemptible [00000000] code: > > > > bash/15687 > > > > caller is debug_smp_processor_id+0x17/0x20 > > > > CPU: 4 PID: 15687 Comm: bash Not tainted 5.19.0-rc7+ #57 > > > > Call Trace: > > > > <TASK> > > > > dump_stack_lvl+0x49/0x63 > > > > dump_stack+0x10/0x16 > > > > check_preemption_disabled+0xdd/0xe0 > > > > debug_smp_processor_id+0x17/0x20 > > > > powerclamp_set_cur_state+0x7f/0xf9 [intel_powerclamp] > > > > ... > > > > ... > > > > > > > > Here CPU 0 is the control CPU by default and changed to the > > > > current CPU, > > > > if CPU 0 offlined. This check has to be performed under > > > > cpus_read_lock(), > > > > hence the above warning. > > > > > > > > Use get_cpu() instead of smp_processor_id() to avoid this BUG. > > > > > > This has exactly the same problem as smp_processor_id(), you just > > > worked around the warning. If it is okay that control_cpu > > > contains > > > stale value, could we have a comment explaining why? > > > > > May I know why does control_cpu have stale value? The control_cpu > > is a random picked online CPU which will be used later to collect > > statistics. > > As long as the control_cpu is online, it is valid IMO. > > So this is confusing, because the code makes the impression that > getting the number of the CPU running the code matters in some way, > which isn't the case. > > Something like cpumask_first(cpu_online_mask) should work as well if > I'm not mistaken and it would be less confusing to use this instead > IMO. That should work as we are under hotplug lock anyway here. Thanks, Srinivas
