Re: [ANNOUNCE] 3.12.6-rt9

[Muli Baron <muli.baron@gmail.com>]

On 21/1/2014 04:17, Steven Rostedt wrote:
> On Sat, 18 Jan 2014 04:15:29 +0100
> Mike Galbraith <bitbucket@online.de> wrote:
>
>
>>> So you also have the timers-do-not-raise-softirq-unconditionally.patch?
>>
>
> People have been complaining that the latest 3.12-rt does not boot on
> intel i7 boxes. And by reverting this patch, it boots fine.
>
> I happen to have a i7 box to test on, and sure enough, the latest
> 3.12-rt locks up on boot and reverting the
> timers-do-not-raise-softirq-unconditionally.patch, it boots fine.
>
> Looking into it, I made this small update, and the box boots. Seems
> checking "active_timers" is not enough to skip raising softirqs. I
> haven't looked at why yet, but I would like others to test this patch
> too.
>
> I'll leave why this lets i7 boxes boot as an exercise for Thomas ;-)
>
> -- Steve
>
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
>
> diff --git a/kernel/timer.c b/kernel/timer.c
> index 46467be..8212c10 100644
> --- a/kernel/timer.c
> +++ b/kernel/timer.c
> @@ -1464,13 +1464,11 @@ void run_local_timers(void)
>   		raise_softirq(TIMER_SOFTIRQ);
>   		return;
>   	}
> -	if (!base->active_timers)
> -		goto out;
>
>   	/* Check whether the next pending timer has expired */
>   	if (time_before_eq(base->next_timer, jiffies))
>   		raise_softirq(TIMER_SOFTIRQ);
> -out:
> +
>   	rt_spin_unlock_after_trylock_in_irq(&base->lock);
>
>   }
> --
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>

While this might fix booting on i7 machines it kinds of defeats the 
original purpose of this patch, which was to let NO_HZ_FULL work 
properly with threaded interrupts. With the active_timers check removed 
the timer interrupt keeps firing even though there is only one task 
running on a specific processor, since it can't shut down the tick 
because the ksoftirqd thread keeps getting scheduled (see the previous 
thread "CONFIG_NO_HZ_FULL + CONFIG_PREEMPT_RT_FULL = nogo" for the full 
discussion).

-- Muli