Re: [Patch v3 5/9] KVM: x86/mmu: Allocate TDP page table's page on correct NUMA node on split

[David Matlack]

On Wed, Dec 21, 2022 at 06:34:53PM -0800, Vipin Sharma wrote:
> When dirty log is enabled, huge pages are split. Page table's pages
> during the split are allocated based on the current thread NUMA node or
> mempolicy. This causes inefficient page table accesses if underlying
> page is on a different NUMA node
> 
> Allocate page table's pages on the same NUMA node as the underlying huge
> page when dirty log is enabled and huge pages are split.
> 
> The performance gain during the pre-copy phase of live migrations of a
> 416 vCPUs and 11 TiB memory VM  on a 8 node host was seen in the range
> of 130% to 150%.

Can you be more specific about this. "The performance" is vague. I know
it's an internal workload and fully explaining it would be difficult,
but you can give readers a slightly more specific idea of what improved.
e.g.

 When testing with a synthetic write-heavy workload in a 416 vCPU VM on
 an 8 NUMA node host, the throughput increased by 150% from X to Y
 operations per second.

It's also necessary to characterize the improvement relative to the
performance when dirty logging is not enabled. Whithout that information
it would be hard for an unfamiliar reader to understand how useful this
change really is.

For example, let's say the throughput of your workload is 100,000
operations per second before dirty logging is enabled, and that drops
down to 1,000 operations per second after dirty logging is enabled. This
commit could increase that by 150% to 2,500 operations per second, but
that's actually not a very meaningful improvement since, either way,
guest performance is degraded by 95+% during dirty logging.

On the other hand, if performance goes from 100,000 to 30,000 normally,
and this commit increases that 30,000 to 75,000 (150%), that's a much
more meaningful improvement.

> 
> Suggested-by: David Matlack <dmatlack@xxxxxxxxxx>
> Signed-off-by: Vipin Sharma <vipinsh@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 12 ++++++++----
>  include/linux/kvm_host.h   | 18 ++++++++++++++++++
>  2 files changed, 26 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 4974fa96deff..376b8dceb3f9 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1403,7 +1403,7 @@ bool kvm_tdp_mmu_wrprot_slot(struct kvm *kvm,
>  	return spte_set;
>  }
>  
> -static struct kvm_mmu_page *__tdp_mmu_alloc_sp_for_split(gfp_t gfp)
> +static struct kvm_mmu_page *__tdp_mmu_alloc_sp_for_split(int nid, gfp_t gfp)
>  {
>  	struct kvm_mmu_page *sp;
>  
> @@ -1413,7 +1413,8 @@ static struct kvm_mmu_page *__tdp_mmu_alloc_sp_for_split(gfp_t gfp)
>  	if (!sp)
>  		return NULL;
>  
> -	sp->spt = (void *)__get_free_page(gfp);
> +	sp->spt = kvm_mmu_get_free_page(nid, gfp);
> +
>  	if (!sp->spt) {
>  		kmem_cache_free(mmu_page_header_cache, sp);
>  		return NULL;
> @@ -1427,6 +1428,9 @@ static struct kvm_mmu_page *tdp_mmu_alloc_sp_for_split(struct kvm *kvm,
>  						       bool shared)
>  {
>  	struct kvm_mmu_page *sp;
> +	int nid;
> +
> +	nid = kvm_pfn_to_page_table_nid(spte_to_pfn(iter->old_spte));
>  
>  	/*
>  	 * Since we are allocating while under the MMU lock we have to be
> @@ -1437,7 +1441,7 @@ static struct kvm_mmu_page *tdp_mmu_alloc_sp_for_split(struct kvm *kvm,
>  	 * If this allocation fails we drop the lock and retry with reclaim
>  	 * allowed.
>  	 */
> -	sp = __tdp_mmu_alloc_sp_for_split(GFP_NOWAIT | __GFP_ACCOUNT);
> +	sp = __tdp_mmu_alloc_sp_for_split(nid, GFP_NOWAIT | __GFP_ACCOUNT);
>  	if (sp)
>  		return sp;
>  
> @@ -1449,7 +1453,7 @@ static struct kvm_mmu_page *tdp_mmu_alloc_sp_for_split(struct kvm *kvm,
>  		write_unlock(&kvm->mmu_lock);
>  
>  	iter->yielded = true;
> -	sp = __tdp_mmu_alloc_sp_for_split(GFP_KERNEL_ACCOUNT);
> +	sp = __tdp_mmu_alloc_sp_for_split(nid, GFP_KERNEL_ACCOUNT);
>  
>  	if (shared)
>  		read_lock(&kvm->mmu_lock);
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index d48064503b88..a262e15ebd19 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1583,6 +1583,24 @@ void kvm_arch_sync_events(struct kvm *kvm);
>  int kvm_cpu_has_pending_timer(struct kvm_vcpu *vcpu);
>  
>  struct page *kvm_pfn_to_refcounted_page(kvm_pfn_t pfn);
> +
> +/*
> + * Tells the appropriate NUMA node location of the page table's page based on
> + * pfn it will point to.

I know what you are trying to say but the wording is a bit awkward. e.g.
"Tells" instead of "Returns", "location" is redundant, "page table's
page", etc. Suggest this:

/*
 * Returns an appropriate NUMA node on which to allocate a page table that
 * maps @pfn.
 */

> + *
> + * Return the nid of the page if pfn is valid and backed by a refcounted page,
> + * otherwise, return the nearest memory node for the current CPU.

I would just drop this as it's just restating the code, which is already
very readable.

> + */
> +static inline int kvm_pfn_to_page_table_nid(kvm_pfn_t pfn)
> +{
> +	struct page *page = kvm_pfn_to_refcounted_page(pfn);
> +
> +	if (page)
> +		return page_to_nid(page);
> +	else
> +		return numa_mem_id();
> +}
> +
>  bool kvm_is_zone_device_page(struct page *page);
>  
>  struct kvm_irq_ack_notifier {
> -- 
> 2.39.0.314.g84b9a713c41-goog
>