On Tue, 2022-10-11 at 21:22 +0800, Chen Yu wrote:
Hi Pavel, On 2022-10-11 at 13:36:46 +0200, Pavel Machek wrote:
Hi!
From: Srinivas Pandruvada srinivas.pandruvada@linux.intel.com
[ 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.
I am also interested to know why this can be stale. The get_cpu() call disables preemption.
#define get_cpu() ({ preempt_disable(); __smp_processor_id(); })
Even if you change it to call debug_smp_processor_id() instead of __smp_processor_id(), it will still not print warning as preempt_count() will return 1.
If after the preemption is enabled if the CPU is offlined, there are hotplug callbacks to handle.
Thanks, Srinivas
thanks, Chenyu
Thanks, Pavel
+++ b/drivers/thermal/intel_powerclamp.c @@ -519,8 +519,10 @@ static int start_power_clamp(void) /* prefer BSP */ control_cpu = 0; - if (!cpu_online(control_cpu)) - control_cpu = smp_processor_id(); + if (!cpu_online(control_cpu)) { + control_cpu = get_cpu(); + put_cpu(); + } clamping = true; schedule_delayed_work(&poll_pkg_cstate_work, 0); -- 2.35.1
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