From: Alexey Kardashevskiy <aik@ozlabs.ru>
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
Subject: Re: [tip: sched/core] sched/core: Initialize the idle task with preemption disabled
Date: Thu, 8 Jul 2021 00:35:23 +1000 [thread overview]
Message-ID: <de7b1d0f-8767-1b33-2950-576029c0d9f7@ozlabs.ru> (raw)
In-Reply-To: <c30097f3-63b4-6fa0-a369-8f4e20ee1040@roeck-us.net>
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
next prev parent reply other threads:[~2021-07-07 14:35 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-12 9:46 [PATCH] sched: Initialize the idle task with preemption disabled Valentin Schneider
2021-05-12 11:15 ` [tip: sched/core] sched/core: " tip-bot2 for Valentin Schneider
2021-07-06 19:44 ` Guenter Roeck
2021-07-06 23:55 ` Valentin Schneider
2021-07-07 4:06 ` Guenter Roeck
2021-07-07 12:03 ` Frederic Weisbecker
2021-07-07 12:11 ` Valentin Schneider
2021-07-07 14:14 ` Guenter Roeck
2021-07-07 14:35 ` Alexey Kardashevskiy [this message]
2021-07-07 14:57 ` Valentin Schneider
2021-07-07 15:31 ` Guenter Roeck
2021-07-07 16:35 ` Guenter Roeck
2021-07-07 17:31 ` Valentin Schneider
2021-07-08 1:50 ` Alexey Kardashevskiy
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