Message-ID: <533A4F64.4080609@linux.vnet.ibm.com>
Date:	Tue, 01 Apr 2014 11:02:20 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	tglx@...utronix.de
CC:	mingo@...hat.com, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org, peterz@...radead.org, rjw@...ysocki.net,
	paulmck@...ux.vnet.ibm.com, davem@...emloft.net
Subject: Re: [PATCH] tick, broadcast: Prevent false alarm when force mask
 contains offline cpus

On 03/28/2014 02:17 PM, Srivatsa S. Bhat wrote:
> On 03/27/2014 03:44 PM, Preeti U Murthy wrote:
>> On 03/27/2014 11:58 AM, Srivatsa S. Bhat wrote:
>>>
>>> Actually, my suggestion was to remove the dying CPU from the force_mask alone,
>>> in the CPU_DYING notifier. The rest of the cleanup (removing it from the other
>>> masks, moving the broadcast duty to someone else etc can still be done at
>>> the CPU_DEAD stage). Also, note that the CPU which is set in force_mask is
>>> definitely not the one doing the broadcast.
>>>
>>> Basically, my reasoning was this:
>>>
>>> If we look at how the 3 broadcast masks (oneshot, pending and force) are
>>> set and cleared during idle entry/exit, we see this pattern:
>>>
>>> oneshot_mask: This is set at BROADCAST_ENTER and cleared at EXIT.
>>> pending_mask: This is set at tick_handle_oneshot_broadcast and cleared at
>>>               EXIT.
>>> force_mask:   This is set at EXIT and cleared at the next call to
>>>               tick_handle_oneshot_broadcast. (Also, if the CPU is set in this
>>>               mask, the CPU doesn't enter deep idle states in subsequent
>>>               idle durations, and keeps polling instead, until it gets the
>>>               broadcast interrupt).
>>>
>>> What we can derive from this is that force_mask is the only mask that can
>>> remain set across an idle ENTER/EXIT sequence. Both of the other 2 masks
>>> can never remain set across a full idle ENTER/EXIT sequence. And a CPU going
>>> offline certainly goes through EXIT if it had gone through ENTER, before
>>> entering stop_machine().
>>>
>>> That means, force_mask is the only odd one out here, which can remain set
>>> when entering stop_machine() for CPU offline. So that's the only mask that
>>> needs to be cleared separately. The other 2 masks take care of themselves
>>> automatically. So, we can have a CPU_DYING callback which just clears the
>>> dying CPU from the force_mask (and does nothing more). That should work, no?
>>
>> Yep I think this will work. Find the modified patch below:
>>
>> Thanks.
>>
>> Regards
>> Preeti U Murthy
>>
>>
>> tick,broadcast:Clear hotplugged cpu in broadcast masks during CPU_DYING notification
>>
>> From: Preeti U Murthy <preeti@...ux.vnet.ibm.com>
>>
>> Its possible that the tick_broadcast_force_mask contains cpus which are not
>> in cpu_online_mask when a broadcast tick occurs. This could happen under the
>> following circumstance assuming CPU1 is among the CPUs waiting for broadcast.
>>
>> CPU0					CPU1
>>
>> Run CPU_DOWN_PREPARE notifiers
>>
>> Start stop_machine			Gets woken up by IPI to run
>> 					stop_machine, sets itself in
>> 					tick_broadcast_force_mask if the
>> 					time of broadcast interrupt is around
>> 					the same time as this IPI.
>>
>> 					Start stop_machine
>> 					  set_cpu_online(cpu1, false)
>> End stop_machine			End stop_machine
>>
>> Broadcast interrupt
>>   Finds that cpu1 in
>>   tick_broadcast_force_mask is offline
>>   and triggers the WARN_ON in
>>   tick_handle_oneshot_broadcast()
>>
>> Clears all broadcast masks
>> in CPU_DEAD stage.
>>
>> While the hotplugged cpu clears its bit in the tick_broadcast_oneshot_mask
>> and tick_broadcast_pending mask during BROADCAST_EXIT, it *sets* its bit
>> in the tick_broadcast_force_mask if the broadcast interrupt is found to be
>> around the same time as the present time. Today we clear all the broadcast
>> masks and shutdown tick devices in the CPU_DEAD stage. But as shown above
>> the broadcast interrupt could occur before this stage is reached and the
>> WARN_ON() gets triggered when it is found that the tick_broadcast_force_mask
>> contains an offline cpu.
>>
>> This WARN_ON was added to capture scenarios where the broadcast mask, be it
>> oneshot/pending/force_mask contain offline cpus whose tick devices have been
>> removed. But here is a case where we trigger the WARN_ON() when the tick
>> device of the hotplugged cpu is still around but we are delaying the clearing
>> of the broadcast masks. This has not been a problem for
>> tick_broadcastoneshot_mask and tick_broadcast_pending_mask since they get
>> cleared on exit from broadcast.
>>    But since the force_mask gets set at the same time on certain occasions
>> it is necessary to move the clearing of masks to a stage during cpu hotplug
>> before the hotplugged cpu clears itself in the online_mask.
>>
> 
> That last sentence is not entirely accurate. During stop-machine in the CPU
> offline path, the CPU removes itself from the cpu_online_mask at the very
> beginning, in the __cpu_disable() call. Only after that the CPU_DYING notifiers
> are invoked. But the advantage of clearing the CPU from the force_mask at
> the CPU_DYING stage is that no other CPU is "noticing" this event, since
> everybody is busy spinning in stop-machine. So, by the time stop-machine
> completes and the CPU is officially offline, it would have "magically" cleared
> itself from the force_mask as well, making things look very consistent for
> the rest of the CPUs (i.e., an offline CPU will never remain set in the
> force_mask).

Hmm right. Besides this,like we discussed offline, this WARN_ON() is
unlikely to be hit in the scenario described in the change log because
the force_mask will be set only when the broadcast interrupt is expected
to be delivered anytime now. There is time before the hotplugged CPU
clears its bit in the cpu_online_mask for the broadcast interrupt to
arrive and see that the force_mask is still a subset of the cpu_online_mask.

But I still think that this patch would be required for the reason
mentioned in the changelog in the last paragraph.
> 
>> Hence move the clearing of broadcast masks to the CPU_DYING notification stage
>> so that they remain consistent with the cpu_online_mask at the time of
>> broadcast delivery at all times.

   ^^^ this is the reason why we will need to move the clearing of mask
from CPU_DEAD to CPU_DYING stage so as to avoid hitting the WARN_ON()
unnecessarily in some valid scenario in the future.

I can send out a V2 patch with the modified changelog with just the last
paragraph. What do you think?

Thomas, do you think this patch will make sense for the reason mentioned
above?

Thanks

Regards
Preeti U Murthy
>>
> 
> This last paragraph sums it up perfectly.
> 
>> Suggested-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
>> Signed-off-by: Preeti U Murthy <preeti@...ux.vnet.ibm.com>
> 
> You might want to alter the changelog a bit as mentioned above. Other than
> that, everything looks fine to me. (But see one minor whitespace nitpick
> below).
> 
> Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
> 
>> ---
>>  kernel/time/clockevents.c    |    1 +
>>  kernel/time/tick-broadcast.c |   20 +++++++++++++++-----
>>  kernel/time/tick-internal.h  |    3 +++
>>  3 files changed, 19 insertions(+), 5 deletions(-)
>>
> [...]
>> @@ -912,11 +925,8 @@ void tick_shutdown_broadcast_oneshot(unsigned int *cpup)
>>  	cpumask_clear_cpu(cpu, tick_broadcast_pending_mask);
>>  	cpumask_clear_cpu(cpu, tick_broadcast_force_mask);
>>
>> -	broadcast_move_bc(cpu);
>> -
>>  	raw_spin_unlock_irqrestore(&tick_broadcast_lock, flags);
>>  }
>> -
> 
> I guess you removed that newline by mistake. Please add it back, it improves
> readability.
> 
> Regards,
> Srivatsa S. Bhat
> 
>>  /*
>>   * Check, whether the broadcast device is in one shot mode
>>   */

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