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Date:   Mon, 7 Dec 2020 02:14:48 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul McKenney <paulmck@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [patch V2 2/9] irqtime: Make accounting correct on RT

On Mon, Dec 07, 2020 at 01:57:25AM +0100, Thomas Gleixner wrote:
> On Mon, Dec 07 2020 at 01:23, Frederic Weisbecker wrote:
> >> --- a/kernel/sched/cputime.c
> >> +++ b/kernel/sched/cputime.c
> >> @@ -60,7 +60,7 @@ void irqtime_account_irq(struct task_str
> >>  	cpu = smp_processor_id();
> >>  	delta = sched_clock_cpu(cpu) - irqtime->irq_start_time;
> >>  	irqtime->irq_start_time += delta;
> >> -	pc = preempt_count() - offset;
> >> +	pc = irq_count() - offset;
> >
> > There are many preempt_count() users all around waiting for similar issues.
> > Wouldn't it be more reasonable to have current->softirq_disable_cnt just saving
> > the softirq count on context switch?
> 
> There are not that many and all of them need to be looked at.
> 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index d2003a7d5ab5..6c899c35d6ba 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -3469,6 +3469,10 @@ static inline void prepare_task(struct task_struct *next)
> >  
> >  static inline void finish_task(struct task_struct *prev)
> >  {
> > +#ifdef CONFIG_PREEMPT_RT
> > +	prev->softirq_disable_cnt = softirq_count();
> > +	__preempt_count_sub(prev->softirq_disable_cnt);
> > +#endif
> 
> You fundamentaly break RT with that.
> 
> If local_bh_disable() fiddles with the actual preempt_count on RT then
> softirq disabled sections and softirq processing are not longer
> preemtible.
> 
> You asked me in the last round of patches to add a proper argument for
> pulling out the softirq count from preempt_count. Here is the revised
> changelog which you agreed with:
> 
>  "RT requires the softirq processing and local bottomhalf disabled regions to
>   be preemptible. Using the normal preempt count based serialization is
>   therefore not possible because this implicitely disables preemption.
>   ....
>  "
> 
> Full text in patch 1/9.
> 
> According to the above folding of softirq count into the actual preempt
> count cannot work at all.
> 
> The current RT approach just works except for the places which look at
> the raw preempt_count and not using the wrappers. Those places are
> restricted to core code and a pretty small number.
> 
> Trying to do what you suggest would be a major surgery all over the
> place including a complete trainwreck on the highly optimized
> preempt_enable() --> preempt decision.

I suspected it was more complicated than I imagined :-)
Nevermind.

Thanks.

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