Subject: Re: offlining cpus breakage

From: Alexey Kardashevskiy <[email-masked]>
Date: Fri, 16 Jan 2015 13:28:44 +1300

On 01/16/2015 02:22 AM, Preeti U Murthy wrote:
> Hi Alexey,
> 
> Can you let me know if the following patch fixes the issue for you ?
> It did for us on one of our machines that we were investigating on.

This fixes the issue for me as well, thanks!

Tested-by: Alexey Kardashevskiy <[email-masked]>	

> Anton,
> Can you let me know if the patch fixes the odd perf report that you observed?
> This is the latest fix that there is to it.
> 
> -------------------------------START PATCH--------------------------------------
> 
> tick/broadcast-hrtimer : Make movement of broadcast hrtimer robust against cpu offline
> 
> From: Preeti U Murthy <[email-masked]>
> 
> When a cpu on which the broadcast hrtimer is queued goes offline, the hrtimer
> needs to be moved to another cpu. There maybe potential race conditions here,
> which can lead to the hrtimer not being queued on any cpu.
> 
> There was a report on softlockups, with cpus being stuck in deep idle states,
> when smt mode switching was being done, especially on systems with smaller number of
> cpus [1]. This is hard to reproduce on a large machine because the chances that an
> offline cpu was the stand by cpu handling broadcast become lesser. Given the
> current code, the only situation that looks capable of triggering scenarios where
> broadcast IPIs are not delivered, is in the cpu offline path. I am
> at a loss to pin point the precise scenario.
> 
> Therefore to avoid any possible race condition between cpu offline and
> movement of the broadcast hrtimer, this patch ensures that the act of keeping
> track of broadcasting wakeup IPIs is started afresh after a cpu offline operation.
> This is done by reseting the expiry time of the hrtimer to a max value. This
> has to be done in the CPU_DYING phase so that it is visible to all cpus right
> after exiting stop_machine.
> 
> The rationale is that during cpu offline, since all cpus are woken up anyway
> to run stop_machine, we are only required to keep track of broadcasting to cpus
> that henceforth enter deep idle states. This ensures that the broadcast hrtimer
> gets positively queued on a cpu as long as there are cpus in deep idle states.
> 
> It has to be noted that this code is in addition to retaining the logic that we
> have today in the broadcast code on an offline operation. If this logic fails to
> move the broadcast hrtimer due to a race condition we have the following patch to
> handle it right.
> 
> [1]http://linuxppc.10917.n7.nabble.com/offlining-cpus-breakage-td88619.html
> There is no issue in programming the decrementer as was presumed and stated in
> this link.
> 
> Signed-off-by: Preeti U Murthy <[email-masked]>
> ---
>  kernel/time/clockevents.c    |    2 +-
>  kernel/time/tick-broadcast.c |    1 +
>  2 files changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
> index 5544990..f3907c9 100644
> --- a/kernel/time/clockevents.c
> +++ b/kernel/time/clockevents.c
> @@ -568,6 +568,7 @@ int clockevents_notify(unsigned long reason, void *arg)
>  
>  	case CLOCK_EVT_NOTIFY_CPU_DYING:
>  		tick_handover_do_timer(arg);
> +		tick_shutdown_broadcast_oneshot(arg);
>  		break;
>  
>  	case CLOCK_EVT_NOTIFY_SUSPEND:
> @@ -580,7 +581,6 @@ int clockevents_notify(unsigned long reason, void *arg)
>  		break;
>  
>  	case CLOCK_EVT_NOTIFY_CPU_DEAD:
> -		tick_shutdown_broadcast_oneshot(arg);
>  		tick_shutdown_broadcast(arg);
>  		tick_shutdown(arg);
>  		/*
> diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
> index 066f0ec..1f5eda6 100644
> --- a/kernel/time/tick-broadcast.c
> +++ b/kernel/time/tick-broadcast.c
> @@ -677,6 +677,7 @@ static void broadcast_move_bc(int deadcpu)
>  		return;
>  	/* This moves the broadcast assignment to this cpu */
>  	clockevents_program_event(bc, bc->next_event, 1);
> +	bc->next_event.tv64 = KTIME_MAX;
>  }
>  
>  /*
> 
> ---------------------------------------END PATCH-------------------------
> Thanks
> 
> Regards
> Preeti U Murthy
> 
> On 01/07/2015 03:07 PM, Alexey Kardashevskiy wrote:
>> Hi!
>>
>> "ppc64_cpu --smt=off" produces multiple error on the latest upstream kernel
>> (sha1 bdec419):
>>
>> NMI watchdog: BUG: soft lockup - CPU#20 stuck for 23s! [swapper/20:0]
>>
>> or
>>
>> INFO: rcu_sched detected stalls on CPUs/tasks: { 2 7 8 9 10 11 12 13 14 15
>> 16 17 18 19 20 21 22 23 2
>> 4 25 26 27 28 29 30 31} (detected by 6, t=2102 jiffies, g=1617, c=1616,
>> q=1441)
>>
>> and many others, all about lockups
>>
>> I did bisecting and found out that reverting these helps:
>>
>> 77b54e9f213f76a23736940cf94bcd765fc00f40 powernv/powerpc: Add winkle
>> support for offline cpus
>> 7cba160ad789a3ad7e68b92bf20eaad6ed171f80 powernv/cpuidle: Redesign idle
>> states management
>> 8eb8ac89a364305d05ad16be983b7890eb462cc3 powerpc/powernv: Enable Offline
>> CPUs to enter deep idle states
>>
>> btw reverting just two of them produces a compile error.
>>
>> It is pseries_le_defconfig, POWER8 machine:
>> timebase        : 512000000
>> platform        : PowerNV
>> model           : palmetto
>> machine         : PowerNV palmetto
>> firmware        : OPAL v3
>>
>>
>> Please help to fix it. Thanks.
>>
>>
> 

-- 
Alexey