Re: [PATCH 2/6] kvm-s390: use hrtimer for clock wakeup from idle

From: Avi Kivity <avi@redhat.com>
To: ehrhardt@linux.vnet.ibm.com
Cc: kvm@vger.kernel.org,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Carsten Otte <cotte@de.ibm.com>
Subject: Re: [PATCH 2/6] kvm-s390: use hrtimer for clock wakeup from idle
Date: Wed, 06 May 2009 15:10:07 +0300	[thread overview]
Message-ID: <4A017E1F.6030908@redhat.com> (raw)
In-Reply-To: <1241534358-32172-3-git-send-email-ehrhardt@linux.vnet.ibm.com>

ehrhardt@linux.vnet.ibm.com wrote:
> From: Christian Borntraeger <borntraeger@de.ibm.com>
>
> This patch reworks the s390 clock comparator wakeup to hrtimer. The clock
> comparator is a per-cpu value that is compared against the TOD clock. If
> ckc <= TOD an external interrupt 1004 is triggered. Since the clock comparator
> and the TOD clock have a much higher resolution than jiffies we should use
> hrtimers to trigger the wakeup. This speeds up guest nanosleep for small
> values.
>
> Since hrtimers callbacks run in hard-irq context, I added a tasklet to do
> the actual work with enabled interrupts. 
>
>  
> -void kvm_s390_idle_wakeup(unsigned long data)
> +void kvm_s390_tasklet(unsigned long parm)
>  {
> -	struct kvm_vcpu *vcpu = (struct kvm_vcpu *)data;
> +	struct kvm_vcpu *vcpu = (struct kvm_vcpu *) parm;
>  
> -	spin_lock_bh(&vcpu->arch.local_int.lock);
> +	spin_lock(&vcpu->arch.local_int.lock);
>  	vcpu->arch.local_int.timer_due = 1;
>  	if (waitqueue_active(&vcpu->arch.local_int.wq))
>  		wake_up_interruptible(&vcpu->arch.local_int.wq);
> -	spin_unlock_bh(&vcpu->arch.local_int.lock);
> +	spin_unlock(&vcpu->arch.local_int.lock);
>  }
>   

Why can't this be done from the timer context (after adjusting the locks)?

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.