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

[Andrew Jones <drjones@redhat.com>]

On Wed, Jan 20, 2016 at 04:20:03PM +0000, Marc Zyngier wrote:
> On 20/01/16 15:06, Andrew Jones wrote:
> > 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.
> 
> Just tried on Seattle with a 64bit guest, and there is hardly any
> difference indeed. Both host and guest are "mostly" defconfig as well.
> So there is a kernel configuration difference.
> 
> Running my 32bit guest on a 64bit host definitely shows a massive
> difference (with 8 vcpus):
> 
> Without "always-on": ~1200 interrupts per second
> With "always-on": ~50 interrupts per second
> 
> [Head scratching, poking Mark]
> 
> Right, I now know what is going on: The arm64 kernel uses
> tick_setup_hrtimer_broadcast() so that it can still use the arch timer
> as a broadcast timer (forcing one CPU to remain on), while the 32bit
> kernel relies on the presence of a backup timer (sp804 anyone?) or the
> guarantee that the timer cannot go away (always-on).
> 
> This is probably why I'm seeing such a gain with a 32bit guest, and none
> with a 64bit guest (the kernel already does the right thing). As to why
> there is such a difference between the two architectures, this is a
> story for another day...
>

Thanks Marc! I just confirmed with an AArch32 guest using QEMU, and the
patch we've hijacked for this thread. Without the patch I get a megaton
of interrupts (~14000/s). With the patch, after letting the guest chill
for a while, I'm getting ~150/s (8 vcpus).

I guess I don't have a test case for the ACPI code though. afaik we only
have UEFI for AArch64 guests, and we don't have ACPI boot without UEFI.
Or maybe I can hack time_init to remove the tick_setup_hrtimer_broadcast
call?

drew