linux-kernel - Re: [PATCH] kvm: irqchip: Break up high order allocations of kvm_irq_routing

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Message-ID: <554CE3A5.7000101@redhat.com>
Date:	Fri, 08 May 2015 18:26:13 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Joerg Roedel <joro@...tes.org>, Gleb Natapov <gleb@...nel.org>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Joerg Roedel <jroedel@...e.de>,
	Christian Borntraeger <borntraeger@...ibm.com>
Subject: Re: [PATCH] kvm: irqchip: Break up high order allocations of kvm_irq_routing_table



On 08/05/2015 14:31, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@...e.de>
> 
> The allocation size of the kvm_irq_routing_table depends on
> the number of irq routing entries because they are all
> allocated with one kzalloc call.
> 
> When the irq routing table gets bigger this requires high
> order allocations which fail from time to time:
> 
> 	qemu-kvm: page allocation failure: order:4, mode:0xd0
> 
> This patch fixes this issue by breaking up the allocation of
> the table and its entries into individual kzalloc calls.
> These could all be satisfied with order-0 allocations, which
> are less likely to fail.
> 
> The downside of this change is the lower performance, because
> of more calls to kzalloc. But given how often kvm_set_irq_routing
> is called in the lifetime of a guest, it doesn't really
> matter much.

It probably doesn't matter much indeed, but can you time the difference?
 kvm_set_irq_routing is not too frequent, but happens enough often that
we had to use a separate SRCU instance just to speed it up (see commit
719d93cd5f5, kvm/irqchip: Speed up KVM_SET_GSI_ROUTING, 2014-01-16).

Paolo

> Signed-off-by: Joerg Roedel <jroedel@...e.de>
> ---
>  virt/kvm/irqchip.c | 40 ++++++++++++++++++++++++++++++++--------
>  1 file changed, 32 insertions(+), 8 deletions(-)
> 
> diff --git a/virt/kvm/irqchip.c b/virt/kvm/irqchip.c
> index 1d56a90..b56168f 100644
> --- a/virt/kvm/irqchip.c
> +++ b/virt/kvm/irqchip.c
> @@ -33,7 +33,6 @@
>  
>  struct kvm_irq_routing_table {
>  	int chip[KVM_NR_IRQCHIPS][KVM_IRQCHIP_NUM_PINS];
> -	struct kvm_kernel_irq_routing_entry *rt_entries;
>  	u32 nr_rt_entries;
>  	/*
>  	 * Array indexed by gsi. Each entry contains list of irq chips
> @@ -118,11 +117,31 @@ int kvm_set_irq(struct kvm *kvm, int irq_source_id, u32 irq, int level,
>  	return ret;
>  }
>  
> +static void free_irq_routing_table(struct kvm_irq_routing_table *rt)
> +{
> +	int i;
> +
> +	if (!rt)
> +		return;
> +
> +	for (i = 0; i < rt->nr_rt_entries; ++i) {
> +		struct kvm_kernel_irq_routing_entry *e;
> +		struct hlist_node *n;
> +
> +		hlist_for_each_entry_safe(e, n, &rt->map[i], link) {
> +			hlist_del(&e->link);
> +			kfree(e);
> +		}
> +	}
> +
> +	kfree(rt);
> +}
> +
>  void kvm_free_irq_routing(struct kvm *kvm)
>  {
>  	/* Called only during vm destruction. Nobody can use the pointer
>  	   at this stage */
> -	kfree(kvm->irq_routing);
> +	free_irq_routing_table(kvm->irq_routing);
>  }
>  
>  static int setup_routing_entry(struct kvm_irq_routing_table *rt,
> @@ -173,25 +192,29 @@ int kvm_set_irq_routing(struct kvm *kvm,
>  
>  	nr_rt_entries += 1;
>  
> -	new = kzalloc(sizeof(*new) + (nr_rt_entries * sizeof(struct hlist_head))
> -		      + (nr * sizeof(struct kvm_kernel_irq_routing_entry)),
> +	new = kzalloc(sizeof(*new) + (nr_rt_entries * sizeof(struct hlist_head)),
>  		      GFP_KERNEL);
>  
>  	if (!new)
>  		return -ENOMEM;
>  
> -	new->rt_entries = (void *)&new->map[nr_rt_entries];
> -
>  	new->nr_rt_entries = nr_rt_entries;
>  	for (i = 0; i < KVM_NR_IRQCHIPS; i++)
>  		for (j = 0; j < KVM_IRQCHIP_NUM_PINS; j++)
>  			new->chip[i][j] = -1;
>  
>  	for (i = 0; i < nr; ++i) {
> +		struct kvm_kernel_irq_routing_entry *e;
> +
> +		r = -ENOMEM;
> +		e = kzalloc(sizeof(*e), GFP_KERNEL);
> +		if (!e)
> +			goto out;
> +
>  		r = -EINVAL;
>  		if (ue->flags)
>  			goto out;
> -		r = setup_routing_entry(new, &new->rt_entries[i], ue);
> +		r = setup_routing_entry(new, e, ue);
>  		if (r)
>  			goto out;
>  		++ue;
> @@ -209,6 +232,7 @@ int kvm_set_irq_routing(struct kvm *kvm,
>  	r = 0;
>  
>  out:
> -	kfree(new);
> +	free_irq_routing_table(new);
> +
>  	return r;
>  }
> 
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