Re: nohz problem with idle time on old hardware

[Steven Rostedt]

On Wed, 9 Apr 2014 19:21:53 +0530
Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:

> On Thu, Nov 14, 2013 at 1:31 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> > Subject: NOHZ: Check for nohz active instead of nohz enabled
> >
> > RCU and the fine grained idle time accounting functions check
> > tick_nohz_enabled. But that variable is merily telling that NOHZ has
> > been enabled in the config and not been disabled on the command line.
> >
> > But it does not tell anything about nohz being active. That's what all
> > this should check for.
> >
> > Matthew reported, that the idle accounting on his old P1 machine
> > showed bogus values, when he enabled NOHZ in the config and did not
> > disable it on the kernel command line. The reason is that his machine
> > uses (refined) jiffies as a clocksource which explains why the "fine"
> > grained accounting went into lala land, because it depends on when the
> > system goes and leaves idle relative to the jiffies increment.
> >
> > Provide a tick_nohz_active indicator and let RCU and the accounting
> > code use this instead of tick_nohz_enable.
> 
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > @@ -973,7 +968,7 @@ static void tick_nohz_switch_to_nohz(void)
> >         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
> >         ktime_t next;
> >
> > -       if (!tick_nohz_enabled)
> > +       if (!tick_nohz_active)
> >                 return;
> 
> Considering the impressive list of Reviewed-by and people involved
> in this patch, I am not sure I am reading the code well here.
> 
> The above change isn't required as per my understanding. Otherwise
> we will never pass that check. tick_nohz_active is initialized as zero
> and so we will keep on returning for ever and wouldn't be able to set
> it to 1 ever.
> 
> I have a patch to fix it up, but wanted to know your opinion before
> sending it.

Ouch! You are correct, this part of the patch makes no sense. That's
what I get for reviewing a patch and not looking at all the code around
the changes. (another kernel developer hangs head in shame :-( )

I think that if statement should be nuked.

-- Steve

> 
> >         local_irq_disable();
> > @@ -981,7 +976,7 @@ static void tick_nohz_switch_to_nohz(void)
> >                 local_irq_enable();
> >                 return;
> >         }
> > -
> > +       tick_nohz_active = 1;
> >         ts->nohz_mode = NOHZ_MODE_LOWRES;
> >
> >         /*

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