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

[Frederic Weisbecker <fweisbec@gmail.com>]

On Mon, Jun 27, 2016 at 07:31:29PM -0400, Rik van Riel wrote:
> On Tue, 2016-06-28 at 01:21 +0200, Frederic Weisbecker wrote:
> > On Wed, Jun 22, 2016 at 10:25:48PM -0400, riel@redhat.com wrote:
> > > 
> > > From: Rik van Riel <riel@redhat.com>
> > > 
> > > 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.

Indeed.

> > > 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?

Yeah that would be better.

Thanks.