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Date: Tue, 25 Aug 2015 06:33:51 +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 2/3] KVM: dynamise halt_poll_ns adjustment
Hi David，
On 8/25/15 1:00 AM, David Matlack wrote:
> On Mon, Aug 24, 2015 at 5:53 AM, Wanpeng Li <wanpeng.li@...mail.com> wrote:
>> There are two new kernel parameters for changing the halt_poll_ns:
>> halt_poll_ns_grow and halt_poll_ns_shrink. halt_poll_ns_grow affects
>> halt_poll_ns when an interrupt arrives and halt_poll_ns_shrink
>> does it when idle VCPU is detected.
>>
>> halt_poll_ns_shrink/ |
>> halt_poll_ns_grow | interrupt arrives | idle VCPU is detected
>> ---------------------+----------------------+-------------------
>> < 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
>>
>> 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.
> I like the idea of growing and shrinking halt_poll_ns, but I'm not sure
> we grow and shrink in the right places here. For example, if vcpu->halt_poll_ns
> gets down to 0, I don't see how it can then grow back up.
This has already done in __grow_halt_poll_ns().
>
> This might work better:
> 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
>
> where halt_poll_ns_max is something reasonable, like 2 millisecond.
>
> You get diminishing returns from halt polling as the length of the
> halt gets longer (halt polling only reduces halt latency by 10-15 us).
> So there's little benefit to polling longer than a few milliseconds.
Great idea, David! Thanks for your review and I will implement it in
next version. :-)
Regards,
Wanpeng Li
>
>> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
>> ---
>> virt/kvm/kvm_main.c | 81 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
>> 1 file changed, 80 insertions(+), 1 deletion(-)
>>
>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> index a122b52..bcfbd35 100644
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -66,9 +66,28 @@
>> 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 \
>> + INT_MAX / KVM_HALT_POLL_NS_GROW
>> +
>> +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 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 . */
>> +int halt_poll_ns_shrink = KVM_HALT_POLL_NS_SHRINK;
>> +module_param(halt_poll_ns_shrink, int, S_IRUGO);
>> +
>> +/* Default is to compute the maximum so we can never overflow. */
>> +unsigned int halt_poll_ns_actual_max = KVM_HALT_POLL_NS_MAX;
>> +unsigned int halt_poll_ns_max = KVM_HALT_POLL_NS_MAX;
>> +module_param(halt_poll_ns_max, int, S_IRUGO);
>> +
>> /*
>> * Ordering of locks:
>> *
>> @@ -1907,6 +1926,62 @@ 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_actual_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)
>> +{
>> + 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)
>> +{
>> + 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);
>> +}
>> +
>> +/*
>> + * halt_poll_ns_actual_max is computed to be one grow_halt_poll_ns() below
>> + * halt_poll_ns_max. (See __grow_halt_poll_ns for the reason.)
>> + * This prevents overflows, because ple_halt_poll_ns is int.
>> + * halt_poll_ns_max effectively rounded down to a multiple of halt_poll_ns_grow in
>> + * this process.
>> + */
>> +static void update_halt_poll_ns_actual_max(void)
>> +{
>> + halt_poll_ns_actual_max =
>> + __shrink_halt_poll_ns(max(halt_poll_ns_max, halt_poll_ns),
>> + halt_poll_ns_grow, INT_MIN);
>> +}
>> +
>> static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
>> {
>> if (kvm_arch_vcpu_runnable(vcpu)) {
>> @@ -1941,6 +2016,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>> */
>> if (kvm_vcpu_check_block(vcpu) < 0) {
>> ++vcpu->stat.halt_successful_poll;
>> + grow_halt_poll_ns(vcpu);
>> goto out;
>> }
>> cur = ktime_get();
>> @@ -1954,6 +2030,8 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>> break;
>>
>> waited = true;
>> + if (halt_poll_ns)
>> + shrink_halt_poll_ns(vcpu);
>> schedule();
>> }
>>
>> @@ -2950,6 +3028,7 @@ static void hardware_enable_nolock(void *junk)
>> cpumask_set_cpu(cpu, cpus_hardware_enabled);
>>
>> r = kvm_arch_hardware_enable();
>> + update_halt_poll_ns_actual_max();
>>
>> if (r) {
>> cpumask_clear_cpu(cpu, cpus_hardware_enabled);
>> --
>> 1.9.1
>>
>> --
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