Re: [PATCH] PM: EM: Fix late boot with holes in CPU topology

[Christian Loehle <christian.loehle@arm.com>]

On 9/1/25 20:47, Rafael J. Wysocki wrote:
> On Mon, Sep 1, 2025 at 7:41 PM Rafael J. Wysocki <rafael@kernel.org> wrote:
>>
>> On Mon, Sep 1, 2025 at 7:33 PM Christian Loehle
>> <christian.loehle@arm.com> wrote:
>>>
>>> On 9/1/25 17:58, Rafael J. Wysocki wrote:
>>>> On Sun, Aug 31, 2025 at 11:44 PM Christian Loehle
>>>> <christian.loehle@arm.com> wrote:
>>>>>
>>>>> commit e3f1164fc9ee ("PM: EM: Support late CPUs booting and capacity
>>>>> adjustment") added a mechanism to handle CPUs that come up late by
>>>>> retrying when any of the `cpufreq_cpu_get()` call fails.
>>>>>
>>>>> However, if there are holes in the CPU topology (offline CPUs, e.g.
>>>>> nosmt), the first missing CPU causes the loop to break, preventing
>>>>> subsequent online CPUs from being updated.
>>>>> Instead of aborting on the first missing CPU policy, loop through all
>>>>> and retry if any were missing.
>>>>>
>>>>> Fixes: e3f1164fc9ee ("PM: EM: Support late CPUs booting and capacity adjustment")
>>>>> Suggested-by: Kenneth Crudup <kenneth.crudup@gmail.com>
>>>>> Reported-by: Kenneth Crudup <kenneth.crudup@gmail.com>
>>>>> Closes: https://lore.kernel.org/linux-pm/40212796-734c-4140-8a85-854f72b8144d@panix.com/
>>>>> Cc: stable@vger.kernel.org
>>>>> Signed-off-by: Christian Loehle <christian.loehle@arm.com>
>>>>> ---
>>>>>  kernel/power/energy_model.c | 13 ++++++++-----
>>>>>  1 file changed, 8 insertions(+), 5 deletions(-)
>>>>>
>>>>> diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
>>>>> index ea7995a25780..b63c2afc1379 100644
>>>>> --- a/kernel/power/energy_model.c
>>>>> +++ b/kernel/power/energy_model.c
>>>>> @@ -778,7 +778,7 @@ void em_adjust_cpu_capacity(unsigned int cpu)
>>>>>  static void em_check_capacity_update(void)
>>>>>  {
>>>>>         cpumask_var_t cpu_done_mask;
>>>>> -       int cpu;
>>>>> +       int cpu, failed_cpus = 0;
>>>>>
>>>>>         if (!zalloc_cpumask_var(&cpu_done_mask, GFP_KERNEL)) {
>>>>>                 pr_warn("no free memory\n");
>>>>> @@ -796,10 +796,8 @@ static void em_check_capacity_update(void)
>>>>>
>>>>>                 policy = cpufreq_cpu_get(cpu);
>>>>>                 if (!policy) {
>>>>> -                       pr_debug("Accessing cpu%d policy failed\n", cpu);
>>>>
>>>> I'm still quite unsure why you want to stop printing this message.  It
>>>> is kind of useful to know which policies have had to be retried, while
>>>> printing the number of them really isn't particularly useful.  And
>>>> this is pr_debug(), so user selectable anyway.
>>>>
>>>> So I'm inclined to retain the line above and drop the new pr_debug() below.
>>>>
>>>> Please let me know if this is a problem.
>>>
>>> For nosmt this leads to a lot of prints every seconds, that's all.
>>> I can resend with the pr_debug for every fail, alternatively print a
>>> cpumask.
>>
>> Printing a cpumask might be better, but it would add some complexity
>> only needed for the printing.
>>
>> Maybe it's just better to not print anything at all.
> 
> I've changed the patch to that effect and tentatively applied it, so
> no need to resend if you agree with this modification.
> 
> Thanks!

All good, thanks!
Yeah I was already leaning towards that now anyway.
I think Kenneth's report (which although he hasn't confirmed would be
that these prints were a red herring for him, they are expected) is
at least an indication that these prints might not be that useful after
all.