Re: [PATCH 2/5] nohz,cputime: remove VTIME_GEN vtime irq time code

[Rik van Riel]

On Tue, 2016-06-28 at 01:21 +0200, Frederic Weisbecker wrote:
> On Wed, Jun 22, 2016 at 10:25:48PM -0400, riel@xxxxxxxxxx wrote:
> > 
> > From: Rik van Riel <riel@xxxxxxxxxx>
> > 
> > The CONFIG_VIRT_CPU_ACCOUNTING_GEN irq time tracking code does not
> > appear to currently work right.
> > 
> > On CPUs that are nohz_full, people typically do not assign IRQs.
> Right, but they can still fire. At least one tick per second, plus
> the
> pinned timers, etc...
> 
> > 
> > 
> > On the housekeeping CPU (when a system is booted up with
> > nohz_full),
> > sampling should work ok to determine irq and softirq time use, but
> > that only covers the housekeeping CPU itself, not the other
> > non-nohz_full CPUs.
> Hmm, every non-nohz_full CPUs, including the CPU 0, account the
> irqtime
> the same way: through the tick (and therefore can't account much of
> it).
> 
But it will be subtracted from the user time, rather
than the idle time during which the irqs happened.

Furthermore, we might well have 100 jiffies worth of
irq & softirq time on a CPU, and get just 1 jiffy
of userspace time, on systems acting like routers.

> > 
> > 
> > On CPUs that are nohz_idle (the typical way a distro kernel is
> > booted), irq time is not accounted at all while the CPU is idle,
> > due to the lack of timer ticks.
> But as soon as a timer tick fires in idle or afterward, the pending
> irqtime is accounted.
> 
> That said I don't see how it explains why we do the below:
> 
> > 
> > 
> > Remove the VTIME_GEN vtime irq time code. The next patch will
> > allow NO_HZ_FULL kernels to use the IRQ_TIME_ACCOUNTING code.
> I don't get the reason why we are doing this. Now arguably the
> irqtime
> accounting is probably not working as well as before since we
> switched to
> jiffy clock. But I still see some hard irqs accounted when
> account_irq_exit()
> is lucky enough to observe that jiffies changed since the beginning
> of
> the interrupt.
> 
> So it's not entirely broken. I agree that we need to switch it to the
> generic irqtime accounting code but breaking the code now to
> reactivate it
> in a subsequent patch is prone to future bisection issues.

Want me to merge patches 2 & 3 into one, so we immediately
start using the generic code and do not run into bisect
issues?

-- 
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