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

[Andrew Jones]

On Wed, Jan 20, 2016 at 02:28:05PM +0000, Marc Zyngier wrote:
> On 20/01/16 14:01, Andrew Jones wrote:
> > 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.
> 
> I just run a couple of quick tests, measuring interrupt rate (vmstat 1)
> on the host, with one VM (2 vcpus) idling, and I'm seeing the following
> thing:
> 
> Without "always-on": ~380 interrupts per second
> With "always-on": ~40 interrupts per second
> 
> This is with kvmtool, 32bit host (but none of that is arch specific anyway).
>

For me (64bit host, one VM (8 vcpus)) of 100 'vmstat 1' samples I have the
following.

Without "always-on": mean=56.370 sd=33.404 min=1 max=244
With "always-on":    mean=51.580 sd=33.361 min=1 max=273

I'm also using kvmtool, and my guest is idle.

So a difference between 32 and 64bit hosts? Again, my guest config is
now just a defconfig. My host config is not, but I'm not sure what
options to look for other than what I wrote above, which are the same
for my host.

Thanks,
drew