Re: [PATCH] sched/topology: clear freecpu bit on detach

[Florian Fainelli <f.fainelli@gmail.com>]

Moving CC list to To

On 5/2/25 06:02, Juri Lelli wrote:
> Hi,
> 
> On 29/04/25 10:15, Florian Fainelli wrote:
>>
>>
>> On 4/22/2025 9:48 PM, Doug Berger wrote:
>>> There is a hazard in the deadline scheduler where an offlined CPU
>>> can have its free_cpus bit left set in the def_root_domain when
>>> the schedutil cpufreq governor is used. This can allow a deadline
>>> thread to be pushed to the runqueue of a powered down CPU which
>>> breaks scheduling. The details can be found here:
>>> https://lore.kernel.org/lkml/20250110233010.2339521-1-opendmb@gmail.com
>>>
>>> The free_cpus mask is expected to be cleared by set_rq_offline();
>>> however, the hazard occurs before the root domain is made online
>>> during CPU hotplug so that function is not invoked for the CPU
>>> that is being made active.
>>>
>>> This commit works around the issue by ensuring the free_cpus bit
>>> for a CPU is always cleared when the CPU is removed from a
>>> root_domain. This likely makes the call of cpudl_clear_freecpu()
>>> in rq_offline_dl() fully redundant, but I have not removed it
>>> here because I am not certain of all flows.
>>>
>>> It seems likely that a better solution is possible from someone
>>> more familiar with the scheduler implementation, but this
>>> approach is minimally invasive from someone who is not.
>>>
>>> Signed-off-by: Doug Berger <opendmb@gmail.com>
>>> ---
>>
>> FWIW, we were able to reproduce this with the attached hotplug.sh script
>> which would just randomly hot plug/unplug CPUs (./hotplug.sh 4). Within a
>> few hundred of iterations you could see the lock up occur, it's unclear why
>> this has not been seen by more people.
>>
>> Since this is not the first posting or attempt at fixing this bug [1] and we
>> consider it to be a serious one, can this be reviewed/commented on/applied?
>> Thanks!
>>
>> [1]: https://lkml.org/lkml/2025/1/14/1687
> 
> So, going back to the initial report, the thing that makes me a bit
> uncomfortable with the suggested change is the worry that it might be
> plastering over a more fundamental issue. Not against it, though, and I
> really appreciate Doug's analysis and proposed fixes!
> 
> Doug wrote:
> 
> "Initially, CPU0 and CPU1 are active and CPU2 and CPU3 have been
> previously offlined so their runqueues are attached to the
> def_root_domain.
> 1) A hot plug is initiated on CPU2.
> 2) The cpuhp/2 thread invokes the cpufreq governor driver during
>     the CPUHP_AP_ONLINE_DYN step.
> 3) The sched util cpufreq governor creates the "sugov:2" thread to
>     execute on CPU2 with the deadline scheduler.
> 4) The deadline scheduler clears the free_cpus mask for CPU2 within
>     the def_root_domain when "sugov:2" is scheduled."
> 
> I wonder if it's OK to schedule sugov:2 on a CPU that didn't reach yet
> complete online state. Peter, others, what do you think?

Peter, can you please review this patch? Thank you
-- 
Florian