Re: [PATCH] tick, broadcast: Prevent false alarm when force mask contains offline cpus

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 03/26/2014 09:26 AM, Preeti U Murthy wrote:
> 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.
> 
> 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 in a valid scenario.
> 
> One could argue that the scenario is invalid and ought to be warned against
> because ideally the broadcast masks need to be cleared of the cpus about to
> go offine before clearing them in the online_mask so that we dont hit these
> scenarios.
> 
> This would mean clearing the masks in CPU_DOWN_PREPARE stage.

Not necessarily. We could clear the mask in the CPU_DYING stage. That way,
offline CPUs will automatically get cleared from the force_mask and hence
the tick-broadcast code will not need to have a special case to deal with
this scenario. What do you think?

Regards,
Srivatsa S. Bhat

> ---
> 
>  kernel/time/tick-broadcast.c |    7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
> index 63c7b2d..30b8731 100644
> --- a/kernel/time/tick-broadcast.c
> +++ b/kernel/time/tick-broadcast.c
> @@ -606,7 +606,12 @@ again:
>  	 */
>  	cpumask_clear_cpu(smp_processor_id(), tick_broadcast_pending_mask);
> 
> -	/* Take care of enforced broadcast requests */
> +	/* Take care of enforced broadcast requests. We could have offline
> +	 * cpus in the tick_broadcast_force_mask. Thats ok, we got the interrupt
> +	 * before we could clear the mask.
> +	 */
> +	cpumask_and(tick_broadcast_force_mask,
> +			tick_broadcast_force_mask, cpu_online_mask);
>  	cpumask_or(tmpmask, tmpmask, tick_broadcast_force_mask);
>  	cpumask_clear(tick_broadcast_force_mask);
> 
>