linux-kernel - Re: [PATCH 9/9] KVM: VMX: automatic PLE window maximum

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Message-ID: <53F44BCD.1010203@redhat.com>
Date:	Wed, 20 Aug 2014 09:18:37 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>,
	kvm@...r.kernel.org
CC:	linux-kernel@...r.kernel.org, Gleb Natapov <gleb@...nel.org>,
	Raghavendra KT <raghavendra.kt@...ux.vnet.ibm.com>,
	Vinod Chegu <chegu_vinod@...com>, Hui-Zhi <hui-zhi.zhao@...com>
Subject: Re: [PATCH 9/9] KVM: VMX: automatic PLE window maximum

Il 20/08/2014 09:16, Paolo Bonzini ha scritto:
> Il 19/08/2014 22:35, Radim Krčmář ha scritto:
>> Every increase of ple_window_grow creates potential overflows.
>> They are not serious, because we clamp ple_window and userspace is
>> expected to fix ple_window_max within a second.
>> ---
>>  arch/x86/kvm/vmx.c | 34 +++++++++++++++++++++++++++++++++-
>>  1 file changed, 33 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index d7f58e8..6873a0b 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -138,7 +138,9 @@ module_param(ple_window, int, S_IRUGO | S_IWUSR);
>>  
>>  /* Default doubles per-vcpu window every exit. */
>>  static int ple_window_grow = KVM_VMX_DEFAULT_PLE_WINDOW_GROW;
>> -module_param(ple_window_grow, int, S_IRUGO | S_IWUSR);
>> +static struct kernel_param_ops ple_window_grow_ops;
>> +module_param_cb(ple_window_grow, &ple_window_grow_ops,
>> +                &ple_window_grow, S_IRUGO | S_IWUSR);
>>  
>>  /* Default resets per-vcpu window every exit to ple_window. */
>>  static int ple_window_shrink = KVM_VMX_DEFAULT_PLE_WINDOW_SHRINK;
>> @@ -5717,6 +5719,36 @@ static void type##_ple_window(struct kvm_vcpu *vcpu) \
>>  make_ple_window_modifier(grow,   *, +) /* grow_ple_window */
>>  make_ple_window_modifier(shrink, /, -) /* shrink_ple_window */
>>  
>> +static void clamp_ple_window_max(void)
>> +{
>> +	int maximum;
>> +
>> +	if (ple_window_grow < 1)
>> +		return;
>> +
>> +	if (ple_window_grow < ple_window)
>> +		maximum = INT_MAX / ple_window_grow;
>> +	else
>> +		maximum = INT_MAX - ple_window_grow;
>> +
>> +	ple_window_max = clamp(ple_window_max, ple_window, maximum);
>> +}
> 
> I think avoiding overflows is better.  In fact, I think you should call
> this function for ple_window_max too.
> 
> You could keep the ple_window_max variable to the user-set value.
> Whenever ple_window_grow or ple_window_max are changed, you can set an
> internal variable (let's call it ple_window_actual_max, but I'm not wed
> to this name) to the computed value, and then do:
> 
> 	if (ple_window_grow < 1 || ple_window_actual_max < ple_window)
> 		new = ple_window;
> 	else if (ple_window_grow < ple_window)
> 		new = max(ple_window_actual_max, old) * ple_window_grow;
> 	else
> 		new = max(ple_window_actual_max, old) + ple_window_grow;

Ehm, this should of course be "min".

Paolo

> (I think the || in the first "if" can be eliminated with some creativity
> in clamp_ple_window_max).
> 
> Paolo
> 
>> +static int set_ple_window_grow(const char *arg, const struct kernel_param *kp)
>> +{
>> +	int ret;
>> +
>> +	clamp_ple_window_max();
>> +	ret = param_set_int(arg, kp);
>> +
>> +	return ret;
>> +}
>> +
>> +static struct kernel_param_ops ple_window_grow_ops = {
>> +	.set = set_ple_window_grow,
>> +	.get = param_get_int,
>> +};
>> +
>>  /*
>>   * Indicate a busy-waiting vcpu in spinlock. We do not enable the PAUSE
>>   * exiting, so only get here on cpu with PAUSE-Loop-Exiting.
>>
> 
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