Re: [Qemu-devel] [PATCH] hw/arm/virt: Add always-on property to the virt board timer

[Andrew Jones <drjones@redhat.com>]

On Tue, Jan 19, 2016 at 07:48:14PM +0100, Andrew Jones wrote:
> On Tue, Jan 19, 2016 at 01:43:07PM +0000, Marc Zyngier wrote:
> > >> On Tue, Jan 19, 2016 at 01:37:16PM +0100, Andrew Jones wrote:
> > > OK, CCing him. One thing I see is that without this change we're
> > > currently setting the clock feature CLOCK_EVT_FEAT_C3STOP, even though
> > > it's not true. Having that set may disable the oneshot capabilityj
> > > necessary to switch to nohz mode? I'll just stop there with my
> > > speculation though, so Marc won't have to correct too much...
> > 
> > You're spot on. See 82a5619 in the kernel tree. When I did a similar
> > change in kvmtool, I saw a massive reduction in the number of timer
> > interrupts injected (specially when the number of vcpu is relatively high).
> > 
> > This also have interesting benefits when running on a model, where
> > you're trying to squeeze the last bits of "performance" from the monster...
> >
> 
> Hmm, I'm probably testing this wrong, but I don't see any difference in
> the number of injected timer interrupts. My guest, which I boot with
> UEFI, has 
> 
> CONFIG_ARM_ARCH_TIMER=y
> CONFIG_ARM_ARCH_TIMER_EVTSTREAM=y
> CONFIG_ARM_TIMER_SP804=y
> CONFIG_HIGH_RES_TIMERS=y
> CONFIG_TICK_ONESHOT=y
> CONFIG_NO_HZ_COMMON=y
> # CONFIG_HZ_PERIODIC is not set
> CONFIG_NO_HZ_IDLE=y
> # CONFIG_NO_HZ_FULL is not set
> CONFIG_NO_HZ=y
> CONFIG_HZ_1000=y
> CONFIG_HZ=1000
> 
> I've boot a guest using DT with and without this patch
> 
> ---WITHOUT---
> 
> # ls /proc/device-tree/timer
> compatible  interrupts  name
> # cat /proc/interrupts                  
>            CPU0       CPU1       CPU2       CPU3       CPU4       CPU5 CPU6       CPU7
>   3:       6958       5766       5166       5187       5576       5129 4695       4398       GIC  27 Edge      arch_timer
> # sleep 120 && cat /proc/interrupts                  
>            CPU0       CPU1       CPU2       CPU3       CPU4       CPU5 CPU6       CPU7
>   3:       7557       5986       5487       5265       6232       5868 5464       4438       GIC  27 Edge      arch_timer
> 
> ---WITH---
> 
> # ls /proc/device-tree/timer
> always-on  compatible  interrupts  name
> # cat /proc/interrupts 
>            CPU0       CPU1       CPU2       CPU3       CPU4       CPU5 CPU6       CPU7
>   3:       7005       6080       4996       5391       5165       5257 4930       4844       GIC  27 Edge      arch_timer
> # sleep 120 && cat /proc/interrupts 
>            CPU0       CPU1       CPU2       CPU3       CPU4       CPU5 CPU6       CPU7
>   3:       7523       6505       5264       6717       5273       5391 5526       4901       GIC  27 Edge      arch_timer
> 
> 
> 
> And kvm trace data has
> 
> ---WITHOUT---
> $ grep kvm_timer_update_irq trace.out | wc -l
> 94336
> ---WITH---
> $ grep kvm_timer_update_irq trace.out | wc -l
> 95838
> 
>

Must be how I'm looking, because I just tried kvmtool with/without
Marc's patch that adds always-on, but don't see any reduction of
interrupts there either. I used a defconfig guest kernel. Also,
not that I think it should matter, but my host kernel is 4.4-rc4
based.

I'd like to be able to see a difference with/without this always-on
patch, not because I don't think we should take it anyway, but because
I need a test case for the ACPI counterpart.

Thanks,
drew