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Re: [Qemu-devel] [PATCH] hw/arm/virt: Add always-on property to the virt
From: |
Marc Zyngier |
Subject: |
Re: [Qemu-devel] [PATCH] hw/arm/virt: Add always-on property to the virt board timer |
Date: |
Wed, 20 Jan 2016 14:28:05 +0000 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Icedove/38.5.0 |
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).
M.
--
Jazz is not dead. It just smells funny...
- [Qemu-devel] [PATCH] hw/arm/virt: Add always-on property to the virt board timer, Christoffer Dall, 2016/01/19
- Re: [Qemu-devel] [PATCH] hw/arm/virt: Add always-on property to the virt board timer, Andrew Jones, 2016/01/19
- Re: [Qemu-devel] [PATCH] hw/arm/virt: Add always-on property to the virt board timer, Christoffer Dall, 2016/01/19
- Re: [Qemu-devel] [PATCH] hw/arm/virt: Add always-on property to the virt board timer, Andrew Jones, 2016/01/19
- Re: [Qemu-devel] [PATCH] hw/arm/virt: Add always-on property to the virt board timer, Marc Zyngier, 2016/01/19
- Re: [Qemu-devel] [PATCH] hw/arm/virt: Add always-on property to the virt board timer, Andrew Jones, 2016/01/19
- Re: [Qemu-devel] [PATCH] hw/arm/virt: Add always-on property to the virt board timer, Andrew Jones, 2016/01/19
- Re: [Qemu-devel] [PATCH] hw/arm/virt: Add always-on property to the virt board timer, Andrew Jones, 2016/01/20
- Re: [Qemu-devel] [PATCH] hw/arm/virt: Add always-on property to the virt board timer,
Marc Zyngier <=
- Re: [Qemu-devel] [PATCH] hw/arm/virt: Add always-on property to the virt board timer, Andrew Jones, 2016/01/20
- Re: [Qemu-devel] [PATCH] hw/arm/virt: Add always-on property to the virt board timer, Marc Zyngier, 2016/01/20
- Re: [Qemu-devel] [PATCH] hw/arm/virt: Add always-on property to the virt board timer, Andrew Jones, 2016/01/20
- Re: [Qemu-devel] [PATCH] hw/arm/virt: Add always-on property to the virt board timer, Marc Zyngier, 2016/01/20
Re: [Qemu-devel] [PATCH] hw/arm/virt: Add always-on property to the virt board timer, Peter Maydell, 2016/01/19