Re: [PATCH] sched,init: Fix DEBUG_PREEMPT vs early boot

[Valentin Schneider]

On 31/05/21 12:21, Peter Zijlstra wrote:
> On Wed, May 19, 2021 at 09:02:34AM -0000, tip-bot2 for Yejune Deng wrote:
>> @@ -19,11 +19,7 @@ unsigned int check_preemption_disabled(const char *what1, const char *what2)
>>      if (irqs_disabled())
>>              goto out;
>>
>> -	/*
>> -	 * Kernel threads bound to a single CPU can safely use
>> -	 * smp_processor_id():
>> -	 */
>> -	if (current->nr_cpus_allowed == 1)
>> +	if (is_percpu_thread())
>>              goto out;
>
> So my test box was unhappy with all this and started spewing lots of
> DEBUG_PREEMPT warns on boot.
>

I get these too, though can't recall getting them when testing the
above. I think it's tied with what Frederic found out with copy_process()
copying PF_NO_SETAFFINITY, which it now no longer does.

> This extends 8fb12156b8db6 to cover the new requirement.
>
> ---
> Subject: sched,init: Fix DEBUG_PREEMPT vs early boot
>
> Extend 8fb12156b8db ("init: Pin init task to the boot CPU, initially")
> to cover the new PF_NO_SETAFFINITY requirement.
>
> While there, move wait_for_completion(&kthreadd_done) into kernel_init()
> to make it absolutely clear it is the very first thing done by the init
> thread.
>
> Fixes: 570a752b7a9b ("lib/smp_processor_id: Use is_percpu_thread() instead of nr_cpus_allowed")
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

Tested-by: Valentin Schneider <valentin.schneider@xxxxxxx>
Reviewed-by: Valentin Schneider <valentin.schneider@xxxxxxx>