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Message-ID: <32b53d3c-b026-fbda-511f-9d2b4274a364@allwinnertech.com>
Date: Mon, 10 Apr 2023 15:06:15 +0800
From: Victor Hassan <victor@...winnertech.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: fweisbec@...il.com, tglx@...utronix.de, mingo@...nel.org,
jindong.yue@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tick/broadcast: Do not set oneshot_mask except
was_periodic was true
On 4/7/2023 2:51 PM, Victor Hassan wrote:
>
>
> On 4/4/2023 8:21 PM, Frederic Weisbecker wrote:
>> On Tue, Apr 04, 2023 at 07:37:06PM +0800, Victor Hassan wrote:
>>>>
>>>> Leading to such race:
>>>>
>>>> * CPU 1 stop its tick, next event is in one hour
>>>> * CPU 0 registers new broadcast and sets CPU 1 in
>>>> tick_broadcast_oneshot_mask
>>>> * CPU 1 runs into cpuidle_enter_state(), and tick_broadcast_enter()
>>>> is ignored because
>>>> the CPU is already in tick_broadcast_oneshot_mask
>>>
>>> Yes.
>>>
>>>> * CPU 1 goes to sleep
>>>> * CPU 0 runs the broadcast callback, sees that the next timer for CPU 1
>>>> is in one hour, program the broadcast to that deadline
>>>> * CPU 1 gets an interrupt that enqueues a new timer expiring in the
>>>> next jiffy
>>>> * CPU 1 don't call tick_broadcast_exit and thus don't remove itself
>>>> from
>>>> tick_broadcast_oneshot_mask
>>>
>>> I'm not sure about this... Actually, I believe CPU 1 *will* call
>>> tick_broadcast_exit in this condition because I cannot find a
>>> limitation on
>>> this execution path.
>>
>> You're right, what I wrote doesn't make sense. Let me try again:
>>
>> * CPU 1 stop its tick, next event is in one hour. It calls
>> tick_broadcast_enter() and goes to sleep.
>> * CPU 1 gets an interrupt that enqueues a new timer expiring in the
>> next jiffy
>> (note it's not yet actually programmed in the tick device)
>> * CPU 1 call tick_broadcast_exit().
>>
>> * CPU 0 registers new broadcast device and sets CPU 1 in
>> tick_broadcast_oneshot_mask
>>
>> * CPU 0 runs the broadcast callback, sees that the next timer for CPU 1
>> is in one hour (because the recently enqueued timer for CPU 1
>> hasn't been programmed
>> yet), so it programs the broadcast to that 1 hour deadline.
>>
>> * CPU 1 runs tick_nohz_idle_stop_tick() which eventually writes and
>> program
>> dev->next_event to next jiffy
>> * CPU 1 runs into cpuidle_enter_state(), and tick_broadcast_enter() is
>> ignored because
>> the CPU is already in tick_broadcast_oneshot_mask, so the
>> dev->next_event
>> change isn't propagated to broadcast.
>>
>> * CPU 1 goes to sleep for 1 hour.
>
> Hi Frederic,
> Yes, I think that make sense :)
Hi Frederic,
If we have reached a consensus, may I add "Reviewed-by: Frederic"
in the next patch?
>
>
>>
>> Does it make more sense? There might be more simple scenario of course.
>>
>> Thanks.
>
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