Subject: Re: loading amd-pstate-ut kernel module crashes with PREEMPT_RT

From: "Gautham R. Shenoy" <[email-masked]>
Date: Tue, 5 Aug 2025 22:52:13 +0530

Hello Bert,

On Thu, Jul 31, 2025 at 11:23:15AM +0200, Bert Karwatzki wrote:
> When loading the amd-pstate-ut kernel module (which is supposed to test
> the amd-pstate cpufreq driver) the following crash occurs (most of the 
> time the loading has to be repeated several times):

Thank you for reporting this. I haven't run amd-pstate with PREEMPT_RT
yet. But for the following issue to occur, we need the schedutil()
governor's sugov_update_single_perf() (which will eventually acquire
the read side of the cpufreq_driver_lock) to race with some call that
acquires the cpufreq_driver_lock on the write-side, thus forcing the
read-side to take the slowpath which can block on PREEMPT_RT .

The amd-pstate-ut has the amd_pstate_ut_check_driver() which will test
switching between one mode to another. These mode-switches involve
unregistering the driver, and that callpath involves write-acquire of
the cpufreq_driver_lock.

So the race should potentially occur when the following code-path
through the amd_pstate_ut_check_driver()

amd_pstate_ut_check_driver()
|-> amd_pstate_set_mode()
 |-> amd_pstate_update_status()
  |-> cpufreq_unregister_driver()
   |->subsys_interface_unregister()
    |-> cpufreq_remove_dev()
      |-> cpufreq_policy_free(policy-cpu-X)
       |-> write_lock_irqsave(&cpufreq_driver_lock)

running concurrently with the schedutil governor on another CPU.

sysvec_apic_timer_interrupt()
|-> try_to_wake_up()
 |-> ttwu_do_activate()
  |-> enqueue_task_fair()
   |-> enqueue_entity()
    |-> update_load_avg()
     |->sugov_update_single_freq()
      |-> amd_pstate_adjust_perf()
       |->cpufreq_cpu_get(on CPU Y)
        |-> read_lock_irqsave(&cpufreq_driver_lock)

Here, the read_lock_irqsave() has to take a slow-path in the presence
of the writer, which can sleep on PREEMPT_RT. But this is illegal as
this is invoked from the interrupt context.

Can you please check if you can generate the same lock-up with the
following script without relying on the amd-pstate-ut driver?

-------------x8------------------x8------------------------------
#!/bin/bash

while ((1));
do
     echo "disable" > /sys/device/system/cpu/amd_pstate/status;
     echo "guided" >  /sys/device/system/cpu/amd_pstate/status;
done
-------------x8------------------x8------------------------------

> Bert Karwatzki

-- 
Thanks and Regards
gautham.