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Date: Fri, 28 Aug 2015 13:30:49 +0800
From: Wanpeng Li <wanpeng.li@...mail.com>
To: David Matlack <dmatlack@...gle.com>
CC: Paolo Bonzini <pbonzini@...hat.com>,
kvm list <kvm@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] KVM: dynamic halt_poll_ns adjustment
On 8/28/15 12:25 AM, David Matlack wrote:
> On Thu, Aug 27, 2015 at 2:59 AM, Wanpeng Li <wanpeng.li@...mail.com> wrote:
>> Hi David,
>> On 8/26/15 1:19 AM, David Matlack wrote:
>>> Thanks for writing v2, Wanpeng.
>>>
>>> On Mon, Aug 24, 2015 at 11:35 PM, Wanpeng Li <wanpeng.li@...mail.com>
>>> wrote:
>>>> There is a downside of halt_poll_ns since poll is still happen for idle
>>>> VCPU which can waste cpu usage. This patch adds the ability to adjust
>>>> halt_poll_ns dynamically.
>>> What testing have you done with these patches? Do you know if this removes
>>> the overhead of polling in idle VCPUs? Do we lose any of the performance
>>> from always polling?
>>>
>>>> There are two new kernel parameters for changing the halt_poll_ns:
>>>> halt_poll_ns_grow and halt_poll_ns_shrink. A third new parameter,
>>>> halt_poll_ns_max, controls the maximal halt_poll_ns; it is internally
>>>> rounded down to a closest multiple of halt_poll_ns_grow. The shrink/grow
>>>> matrix is suggested by David:
>>>>
>>>> if (poll successfully for interrupt): stay the same
>>>> else if (length of kvm_vcpu_block is longer than halt_poll_ns_max):
>>>> shrink
>>>> else if (length of kvm_vcpu_block is less than halt_poll_ns_max): grow
>>> The way you implemented this wasn't what I expected. I thought you would
>>> time
>>> the whole function (kvm_vcpu_block). But I like your approach better. It's
>>> simpler and [by inspection] does what we want.
>>
>> I see there is more idle vCPUs overhead w/ this method even more than always
>> halt-poll, so I bring back grow vcpu->halt_poll_ns when interrupt arrives
>> and shrinks when idle VCPU is detected. The perfomance looks good in v4.
> Why did this patch have a worse idle overhead than always poll?
I’m not sure. I make a mistake when I report the kernelbuild test, the
perfomance is also worse than always poll w/ your method. I think your
method didn't grow halt_poll_ns according to if interrupt arrives.
Regards,
Wanpeng Li
>
>> Regards,
>> Wanpeng Li
>>
>>
>>>> halt_poll_ns_shrink/ |
>>>> halt_poll_ns_grow | grow halt_poll_ns | shrink halt_poll_ns
>>>> ---------------------+----------------------+-------------------
>>>> < 1 | = halt_poll_ns | = 0
>>>> < halt_poll_ns | *= halt_poll_ns_grow | /= halt_poll_ns_shrink
>>>> otherwise | += halt_poll_ns_grow | -= halt_poll_ns_shrink
>>> I was curious why you went with this approach rather than just the
>>> middle row, or just the last row. Do you think we'll want the extra
>>> flexibility?
>>>
>>>> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
>>>> ---
>>>> virt/kvm/kvm_main.c | 65
>>>> ++++++++++++++++++++++++++++++++++++++++++++++++++++-
>>>> 1 file changed, 64 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>>>> index 93db833..2a4962b 100644
>>>> --- a/virt/kvm/kvm_main.c
>>>> +++ b/virt/kvm/kvm_main.c
>>>> @@ -66,9 +66,26 @@
>>>> MODULE_AUTHOR("Qumranet");
>>>> MODULE_LICENSE("GPL");
>>>>
>>>> -static unsigned int halt_poll_ns;
>>>> +#define KVM_HALT_POLL_NS 500000
>>>> +#define KVM_HALT_POLL_NS_GROW 2
>>>> +#define KVM_HALT_POLL_NS_SHRINK 0
>>>> +#define KVM_HALT_POLL_NS_MAX 2000000
>>> The macros are not necessary. Also, hard coding the numbers in the param
>>> definitions will make reading the comments above them easier.
>>>
>>>> +
>>>> +static unsigned int halt_poll_ns = KVM_HALT_POLL_NS;
>>>> module_param(halt_poll_ns, uint, S_IRUGO | S_IWUSR);
>>>>
>>>> +/* Default doubles per-vcpu halt_poll_ns. */
>>>> +static unsigned int halt_poll_ns_grow = KVM_HALT_POLL_NS_GROW;
>>>> +module_param(halt_poll_ns_grow, int, S_IRUGO);
>>>> +
>>>> +/* Default resets per-vcpu halt_poll_ns . */
>>>> +static unsigned int halt_poll_ns_shrink = KVM_HALT_POLL_NS_SHRINK;
>>>> +module_param(halt_poll_ns_shrink, int, S_IRUGO);
>>>> +
>>>> +/* halt polling only reduces halt latency by 10-15 us, 2ms is enough */
>>> Ah, I misspoke before. I was thinking about round-trip latency. The
>>> latency
>>> of a single halt is reduced by about 5-7 us.
>>>
>>>> +static unsigned int halt_poll_ns_max = KVM_HALT_POLL_NS_MAX;
>>>> +module_param(halt_poll_ns_max, int, S_IRUGO);
>>> We can remove halt_poll_ns_max. vcpu->halt_poll_ns can always start at
>>> zero
>>> and grow from there. Then we just need one module param to keep
>>> vcpu->halt_poll_ns from growing too large.
>>>
>>> [ It would make more sense to remove halt_poll_ns and keep
>>> halt_poll_ns_max,
>>> but since halt_poll_ns already exists in upstream kernels, we probably
>>> can't
>>> remove it. ]
>>>
>>>> +
>>>> /*
>>>> * Ordering of locks:
>>>> *
>>>> @@ -1907,6 +1924,48 @@ void kvm_vcpu_mark_page_dirty(struct kvm_vcpu
>>>> *vcpu, gfn_t gfn)
>>>> }
>>>> EXPORT_SYMBOL_GPL(kvm_vcpu_mark_page_dirty);
>>>>
>>>> +static unsigned int __grow_halt_poll_ns(unsigned int val)
>>>> +{
>>>> + if (halt_poll_ns_grow < 1)
>>>> + return halt_poll_ns;
>>>> +
>>>> + val = min(val, halt_poll_ns_max);
>>>> +
>>>> + if (val == 0)
>>>> + return halt_poll_ns;
>>>> +
>>>> + if (halt_poll_ns_grow < halt_poll_ns)
>>>> + val *= halt_poll_ns_grow;
>>>> + else
>>>> + val += halt_poll_ns_grow;
>>>> +
>>>> + return val;
>>>> +}
>>>> +
>>>> +static unsigned int __shrink_halt_poll_ns(int val, int modifier, int
>>>> minimum)
>>> minimum never gets used.
>>>
>>>> +{
>>>> + if (modifier < 1)
>>>> + return 0;
>>>> +
>>>> + if (modifier < halt_poll_ns)
>>>> + val /= modifier;
>>>> + else
>>>> + val -= modifier;
>>>> +
>>>> + return val;
>>>> +}
>>>> +
>>>> +static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
>>> These wrappers aren't necessary.
>>>
>>>> +{
>>>> + vcpu->halt_poll_ns = __grow_halt_poll_ns(vcpu->halt_poll_ns);
>>>> +}
>>>> +
>>>> +static void shrink_halt_poll_ns(struct kvm_vcpu *vcpu)
>>>> +{
>>>> + vcpu->halt_poll_ns = __shrink_halt_poll_ns(vcpu->halt_poll_ns,
>>>> + halt_poll_ns_shrink, halt_poll_ns);
>>>> +}
>>>> +
>>>> static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
>>>> {
>>>> if (kvm_arch_vcpu_runnable(vcpu)) {
>>>> @@ -1954,6 +2013,10 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>>>> break;
>>>>
>>>> waited = true;
>>>> + if (vcpu->halt_poll_ns > halt_poll_ns_max)
>>>> + shrink_halt_poll_ns(vcpu);
>>>> + else
>>>> + grow_halt_poll_ns(vcpu);
>>> Shouldn't this go after the loop, and before "out:", in case we schedule
>>> more than once? You can gate it on "if (waited)" so it only runs if we
>>> actually scheduled.
>>>
>>>> schedule();
>>>> }
>>>>
>>>> --
>>>> 1.9.1
>>>>
--
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