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

[Frederic Weisbecker <fweisbec@gmail.com>]

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).

So I'm a bit confused by the above statements..

> 
> 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.

Thanks.