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Date: Thu, 27 Aug 2015 17:59:19 +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
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.
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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