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Message-ID: <de7b1d0f-8767-1b33-2950-576029c0d9f7@ozlabs.ru>
Date:   Thu, 8 Jul 2021 00:35:23 +1000
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Subject: Re: [tip: sched/core] sched/core: Initialize the idle task with
 preemption disabled
Content-Language: en-US
To:     Guenter Roeck <linux@roeck-us.net>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Frederic Weisbecker <frederic@kernel.org>
Cc:     linux-kernel@vger.kernel.org, linux-tip-commits@vger.kernel.org,
        Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, x86@kernel.org
References: <20210512094636.2958515-1-valentin.schneider@arm.com>
 <162081815405.29796.14574924529325899839.tip-bot2@tip-bot2>
 <20210706194456.GA1823793@roeck-us.net> <87fswr6lqv.mognet@arm.com>
 <20210707120305.GB115752@lothringen> <87czru727k.mognet@arm.com>
 <c30097f3-63b4-6fa0-a369-8f4e20ee1040@roeck-us.net>
From:   Alexey Kardashevskiy <aik@ozlabs.ru>
In-Reply-To: <c30097f3-63b4-6fa0-a369-8f4e20ee1040@roeck-us.net>
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On 08/07/2021 00:14, Guenter Roeck wrote:
> 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.

I can reproduce the problem on powerpc easily - qemu with "-smp 2" does it.


-- 
Alexey