linux-kernel - Re: [PATCH V3] tick/broadcast: Make movement of broadcast hrtimer robust against hotplug

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Message-ID: <54C8B3D2.3070608@linux.vnet.ibm.com>
Date:	Wed, 28 Jan 2015 15:32:58 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	aik@...abs.ru, shreyas@...ux.vnet.ibm.com,
	LKML <linux-kernel@...r.kernel.org>, michael@...erman.id.au,
	Peter Zijlstra <peterz@...radead.org>,
	Anton Blanchard <anton@...ba.org>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH V3] tick/broadcast: Make movement of broadcast hrtimer
 robust against hotplug

On 01/27/2015 09:01 AM, Preeti U Murthy wrote:
> On 01/22/2015 04:45 PM, Thomas Gleixner wrote:
>> On Thu, 22 Jan 2015, Preeti U Murthy wrote:
>>> On 01/21/2015 05:16 PM, Thomas Gleixner wrote:
>>> How about when the cpu that is going offline receives a timer interrupt
>>> just before setting its state to CPU_DEAD ? That is still possible right
>>> given that its clock devices may not have been shutdown and it is
>>> capable of receiving interrupts for a short duration. Even with the
>>> above patch, is the following scenario possible ?
>>>
>>>                 CPU0                                  CPU1
>>> t0         Receives timer interrupt
>>>
>>> t1         Sees that there are hrtimers
>>>            to be serviced (hrtimers are not yet migrated)
>>>
>>> t2         calls hrtimer_interrupt()
>>>
>>> t3         tick_program_event()                   CPU_DEAD notifiers
>>>                                                 CPU0's td->evtdev = NULL
>>>
>>> t4         clockevent_program_event()
>>>            references NULL tick device pointer
>>>
>>> So my concern is that since the CLOCK_EVT_NOTIFY_CPU_DEAD callback
>>> handles shutting down of devices besides moving tick related duties.
>>> it's functions may race with the hotplug cpu still handling tick events.
>>
>>   __cpu_disable() is supposed to block interrupts on the dying cpu.
>>
>> But I agree, we should make it more robust. So we want an explicit
>> call for disabling the cpu local stuff and an explicit takeover of the
>> broadcast duty. I'm anyway distangling the clockevents_notify() stuff,
>> so it should be simple to do so.

Thomas ping. Would you be posting this patch?
> 
> I noticed that tick_handover_do_timer() function also suffers from the
> issue that the patch I posted for moving the broadcast duty had, in that
> it relies on all cpus participating in stop_machine(). In a design where
> all cpus do not participate in stop_machine(), if the freshly nominated
> do_timer cpu is idle, there is no update of jiffies till that cpu gets
> back to being busy. So we must do an explicit take over of *both* the
> broadcast and do_timer duty just before the CPU_DEAD phase.

Regards
Preeti u Murthy

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