linux-kernel - Re: [PATCH] KVM: halt-polling: poll if emulated lapic timer will fire soon

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Message-ID: <573DA24D.3080507@de.ibm.com>
Date:	Thu, 19 May 2016 13:23:57 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc:	Wanpeng Li <wanpeng.li@...mail.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Radim Krčmář <rkrcmar@...hat.com>,
	David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH] KVM: halt-polling: poll if emulated lapic timer will fire
 soon

On 05/19/2016 11:26 AM, Wanpeng Li wrote:

I think in general a good idea to poll if a timer will expire soon.

Some patch comments:

Same for all non-x86 archs:
> +static inline unsigned int kvm_arch_timer_remaining(struct kvm_vcpu *vcpu) {}

A function returning int, without a return statement? 
That gives at least a compiler warning.

> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -78,6 +78,9 @@ module_param(halt_poll_ns_grow, uint, S_IRUGO | S_IWUSR);
>  static unsigned int halt_poll_ns_shrink;
>  module_param(halt_poll_ns_shrink, uint, S_IRUGO | S_IWUSR);
> 
> +/* lower-end of message passing workload latency TCP_RR's poll time < 10us */
> +static unsigned int halt_poll_ns_base = 10000;
> +
>  /*
>   * Ordering of locks:
>   *
> @@ -1966,7 +1969,7 @@ static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
>  	grow = READ_ONCE(halt_poll_ns_grow);
>  	/* 10us base */
>  	if (val == 0 && grow)
> -		val = 10000;
> +		val = halt_poll_ns_base;
>  	else
>  		val *= grow;
> 
> @@ -2015,11 +2018,15 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>  	DECLARE_SWAITQUEUE(wait);
>  	bool waited = false;
>  	u64 block_ns;
> +	unsigned int delta, remaining;
> 
> +	remaining = kvm_arch_timer_remaining(vcpu);

and now it causes undefined behaviour, no?


>  	start = cur = ktime_get();
> -	if (vcpu->halt_poll_ns) {
> -		ktime_t stop = ktime_add_ns(ktime_get(), vcpu->halt_poll_ns);
> +	if (vcpu->halt_poll_ns || (remaining < halt_poll_ns_base)) {
> +		ktime_t stop;
> 
> +		delta = vcpu->halt_poll_ns ? vcpu->halt_poll_ns : remaining;
> +		stop = ktime_add_ns(ktime_get(), delta);
>  		++vcpu->stat.halt_attempted_poll;
>  		do {
>  			/*
> 

So you avoid to shrink/grow for these cases? Probably makes sense