linux-kernel - Re: [tip: sched/core] sched/core: Initialize the idle task with preemption disabled

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Date:   Wed, 7 Jul 2021 07:14:45 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Valentin Schneider <valentin.schneider@....com>,
        Frederic Weisbecker <frederic@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org
Subject: Re: [tip: sched/core] sched/core: Initialize the idle task with
 preemption disabled

On 7/7/21 5:11 AM, Valentin Schneider wrote:
> On 07/07/21 14:03, Frederic Weisbecker wrote:
>> On Wed, Jul 07, 2021 at 12:55:20AM +0100, Valentin Schneider wrote:
>>> Thanks for the report.
>>>
>>> So somehow the init task ends up with a non-zero preempt_count()? Per
>>> FORK_PREEMPT_COUNT we should exit __ret_from_fork() with a zero count, are
>>> you hitting the WARN_ONCE() in finish_task_switch()?
>>>
>>> Does CONFIG_DEBUG_PREEMPT=y yield anything interesting?
>>>
>>> I can't make sense of this right now, but it's a bit late :) I'll grab some
>>> toolchain+qemu tomorrow and go poke at it (and while at it I need to do the
>>> same with powerpc).
>>
>> One possible issue is that s390's init_idle_preempt_count() doesn't apply on the
>> target idle task but on the _current_ CPU. And since smp_init() ->
>> idle_threads_init() is actually called remotely, we are overwriting the current
>> CPU preempt_count() instead of the target one.
> 
> Indeed, this becomes quite obvious when tracing the preemption count
> changes. This also means that s390 relied on the idle_thread_get() from the
> hotplug machinery to properly setup the preempt count, rather than
> init_idle_preempt_count() - which is quite yuck.
> 
> I'll write a patch for that and likely one for powerpc.
> 

Can you reproduce the problem with a powerpc qemu emulation ?
If so, how do you reproduce it there ? Reason for asking is that I don't see
the problem with any of my powerpc emulations, and I would like to add test
case(s) if possible.

Thanks,
Guenter