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SubjectRe: [PATCH] printk: robustify printk
On Mon, Aug 11, 2008 at 12:45:26PM +0200, Ingo Molnar wrote:
>
> * Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
>
> > On Fri, 2008-08-08 at 21:21 +0200, Peter Zijlstra wrote:
> >
> > > The initial printk_tick() based implementation didn't suffer this
> > > problem, should we revert to that scheme?
> >
> > Just in case people care..
> >
> > ---
> > Subject: printk: robustify printk
> >
> > Avoid deadlocks against rq->lock and xtime_lock by deferring the klogd
> > wakeup by polling from the timer tick.
>
> i missed most of the discussion, but this seems like the simplest (and
> hence ultimately the best) approach to me.
>
> Coupling printk with RCU, albeit elegant, does not seem like the right
> choice to me in this specific case: printk as an essential debug
> mechanism should be as decoupled as possible.
>
> Also, once we accept the possibility of async klogd completion, we might
> as well do it all the time.
>
> i have only one sidenote:
>
> > --- linux-2.6.orig/kernel/time/tick-sched.c
> > +++ linux-2.6/kernel/time/tick-sched.c
> > @@ -255,7 +255,7 @@ void tick_nohz_stop_sched_tick(int inidl
> > next_jiffies = get_next_timer_interrupt(last_jiffies);
> > delta_jiffies = next_jiffies - last_jiffies;
> >
> > - if (rcu_needs_cpu(cpu))
> > + if (rcu_needs_cpu(cpu) || printk_needs_cpu(cpu))
> > delta_jiffies = 1;
>
> this change made a previous design quirks even more visible: these are
> items that are not purely event driven but need some polling component.
> RCU is one, and now printk is another.
>
> We could clean this up further by integrating the rcu_needs_cpu() and
> printk_needs_cpu() into a softirq mechanism. We already check for
> pending softirqs in tick-sched.c, so the above complication would go
> away completely.

I am missing something here. Are you suggesting that RCU call out
when a given CPU has nothing to do, rather than the current behavior
where rcu_needs_cpu() is invoked when a CPU is being considered for
dynticks idle mode? My concern with this approach would be races that
are currently avoided by the fact that calls to rcu_needs_cpu() are
performed with hardirqs disabled.

Thanx, Paul

> ( But that's for a separate cleanup patch i think. )
>
> No strong feelings though. Peter, which one do you prefer?
>
> Ingo


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