On Tue, 2016-06-28 at 01:21 +0200, Frederic Weisbecker wrote: > On Wed, Jun 22, 2016 at 10:25:48PM -0400, r...@redhat.com wrote: > > > > From: Rik van Riel <r...@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. > > > > > > 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? -- All Rights Reversed.
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