linux-kernel - Re: [PATCH] tick/broadcast: Do not set oneshot_mask except was

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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.
>