Re: [PATCH v4 2/4] PM / devfreq: Add cpu based scaling support to passive governor

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On 22. 5. 13. 07:34, Marek Szyprowski wrote:
Hi Chanwoo,

On 11.05.2022 11:35, Chanwoo Choi wrote:
From: Saravana Kannan <skannan@xxxxxxxxxxxxxx>

Many CPU architectures have caches that can scale independent of the
CPUs. Frequency scaling of the caches is necessary to make sure that the
cache is not a performance bottleneck that leads to poor performance and
power. The same idea applies for RAM/DDR.

To achieve this, this patch adds support for cpu based scaling to the
passive governor. This is accomplished by taking the current frequency
of each CPU frequency domain and then adjust the frequency of the cache
(or any devfreq device) based on the frequency of the CPUs. It listens
to CPU frequency transition notifiers to keep itself up to date on the
current CPU frequency.

To decide the frequency of the device, the governor does one of the
following:
* Derives the optimal devfreq device opp from required-opps property of
    the parent cpu opp_table.

* Scales the device frequency in proportion to the CPU frequency. So, if
    the CPUs are running at their max frequency, the device runs at its
    max frequency. If the CPUs are running at their min frequency, the
    device runs at its min frequency. It is interpolated for frequencies
    in between.

Tested-by: Chen-Yu Tsai <wenst@xxxxxxxxxxxx>
Tested-by: Johnson Wang <johnson.wang@xxxxxxxxxxxx>
Signed-off-by: Saravana Kannan <skannan@xxxxxxxxxxxxxx>
[Sibi: Integrated cpu-freqmap governor into passive_governor]
Signed-off-by: Sibi Sankar <sibis@xxxxxxxxxxxxxx>
[Chanwoo: Fix conflict with latest code and cleanup code]
Signed-off-by: Chanwoo Choi <cw00.choi@xxxxxxxxxxx>

This patch landed in today's linux next-20220512 as commit 2ab415d4e4e6
("PM / devfreq: Add cpu based scaling support to passive governor").

It triggers the following NULL pointer dereference on Exynos based boards:

exynos-bus: new bus device registered: soc:bus-leftbus (100000 KHz ~
200000 KHz)
exynos-bus: new bus device registered: soc:bus-rightbus (100000 KHz ~
200000 KHz)
exynos-bus: new bus device registered: soc:bus-display (160000 KHz ~
200000 KHz)
exynos-bus: new bus device registered: soc:bus-fsys (100000 KHz ~ 134000
KHz)
exynos-bus: new bus device registered: soc:bus-peri ( 50000 KHz ~ 100000
KHz)
exynos-bus: new bus device registered: soc:bus-mfc (100000 KHz ~ 200000 KHz)
8<--- cut here ---
Unable to handle kernel NULL pointer dereference at virtual address 0000003c
[0000003c] *pgd=00000000
Internal error: Oops: 5 [#1] PREEMPT SMP ARM
Modules linked in:
CPU: 3 PID: 8 Comm: kworker/u8:0 Not tainted 5.18.0-rc6-next-20220512 #5014
Hardware name: Samsung Exynos (Flattened Device Tree)
Workqueue: devfreq_wq devfreq_monitor
PC is at __mutex_lock+0x48/0x948
LR is at lock_is_held_type+0x104/0x1a4
pc : [<c0b93098>]    lr : [<c0b8f2ec>]    psr: 60000053
sp : f0889dc0  ip : 600000d3  fp : c1dca624
r10: c1dca44c  r9 : 00000000  r8 : c1984104
r7 : c1d7f000  r6 : 00000000  r5 : 00000001  r4 : 00000008
r3 : 00000000  r2 : 00000000  r1 : 2de44000  r0 : 00000000
Flags: nZCv  IRQs on  FIQs off  Mode SVC_32  ISA ARM  Segment none
Control: 10c5387d  Table: 4000404a  DAC: 00000051
Register r0 information: NULL pointer
Register r1 information: non-paged memory
Register r2 information: NULL pointer
Register r3 information: NULL pointer
Register r4 information: non-paged memory
Register r5 information: non-paged memory
Register r6 information: NULL pointer
Register r7 information: slab task_struct start c1d7f000 pointer offset 0
Register r8 information: non-slab/vmalloc memory
Register r9 information: NULL pointer
Register r10 information: slab kmalloc-2k start c1dca000 pointer offset
1100 size 2048
Register r11 information: slab kmalloc-2k start c1dca000 pointer offset
1572 size 2048
Register r12 information: non-paged memory
Process kworker/u8:0 (pid: 8, stack limit = 0x(ptrval))
Stack: (0xf0889dc0 to 0xf088a000)
...
   __mutex_lock from mutex_lock_nested+0x1c/0x24
   mutex_lock_nested from devfreq_passive_notifier_call+0x24/0x90
   devfreq_passive_notifier_call from srcu_notifier_call_chain+0x98/0x114
   srcu_notifier_call_chain from devfreq_set_target+0x6c/0x304
   devfreq_set_target from devfreq_update_target+0x98/0xe8
   devfreq_update_target from devfreq_monitor+0x28/0x1c0
   devfreq_monitor from process_one_work+0x288/0x774
   process_one_work from worker_thread+0x44/0x504
   worker_thread from kthread+0xf4/0x128
   kthread from ret_from_fork+0x14/0x2c
Exception stack(0xf0889fb0 to 0xf0889ff8)
...
---[ end trace 0000000000000000 ]---

The issue is caused by the lack of setting devfreq_passive_data->this
pointer in devfreq_passive_register_notifier. However, after adding:

@@ -395,6 +395,9 @@ static int devfreq_passive_register_notifier(struct
devfreq *devfreq)
          if (!parent)
                  return -EPROBE_DEFER;

+       if (!p_data->this)
+               p_data->this = devfreq;
+
          nb->notifier_call = devfreq_passive_notifier_call;
          return devfreq_register_notifier(parent, nb,
DEVFREQ_TRANSITION_NOTIFIER);
   }

the NULL pointer dereference is gone, but I see the following warnings
on Odroid U3 board, which were not present before this patch:

devfreq soc:bus-acp: failed to update devfreq using passive governor
devfreq soc:bus-c2c: failed to update devfreq using passive governor
devfreq soc:bus-acp: failed to update devfreq using passive governor
devfreq soc:bus-c2c: failed to update devfreq using passive governor

Hi Marek,

Thanks for the report. I'll fix it.

--
Best Regards,
Samsung Electronics
Chanwoo Choi



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