Re: [PATCH] KVM: arm64: Limit stage2_apply_range() batch size to smallest block

[Oliver Upton]

Hi Krister,

On Thu, Mar 28, 2024 at 12:05:08PM -0700, Krister Johansen wrote:
> stage2_apply_range() for unmap operations can interfere with the
> performance of IO if the device's interrupts share the CPU where the
> unmap operation is occurring.  commit 5994bc9e05c2 ("KVM: arm64: Limit
> stage2_apply_range() batch size to largest block") improved this.  Prior
> to that commit, workloads that were unfortunate enough to have their IO
> interrupts pinned to the same CPU as the unmap operation would observe a
> complete stall.  With the switch to using the largest block size, it is
> possible for IO to make progress, albeit at a reduced speed.

Can you describe the workload a bit more? I'm having a hard time
understanding how you're unmapping that much memory on the fly in
your workload. Is guest memory getting swapped? Are VMs being torn
down?

Also, it seems a bit odd to steer interrupts *into* the workload you
care about...

> Further reducing the stage2_apply_range() batch size has substantial
> performance improvements for IO that share a CPU performing an unmap
> operation.  By switching to a 2mb chunk, IO performance regressions were
> no longer observed in this author's tests.  E.g. it was possible to
> obtain the advertised device throughput despite an unmap operation
> occurring on the CPU where the interrupt was running.  There is a
> tradeoff, however.  No changes were observed in per-operation timings
> when running the kvm_pagetable_test without an interrupt load.  However,
> with a 64gb VM, 1 vcpu, and 4k pages and a IO load, map times increased
> by about 15% and unmap times increased by about 58%.  In essence, this
> trades slower map/unmap times for improved IO throughput.

There are other users of the range-based operations, like
write-protection. Live migration is especially sensitive to the latency
of page table updates as it can affect the VMM's ability to converge
with the guest.

> Cc: <stable@xxxxxxxxxxxxxxx> # 5.15.x: 3b5c082bbfa2: KVM: arm64: Work out supported block level at compile time
> Cc: <stable@xxxxxxxxxxxxxxx> # 5.15.x: 5994bc9e05c2: KVM: arm64: Limit stage2_apply_range() batch size to largest block
> Cc: <stable@xxxxxxxxxxxxxxx> # 5.15.x

This is a performance improvement, *not* a correctness fix. Please don't
cc stable for it.

> Suggested-by: Ali Saidi <alisaidi@xxxxxxxxxx>
> Signed-off-by: Krister Johansen <kjlx@xxxxxxxxxxxxxxxxxx>
> ---
>  arch/arm64/include/asm/kvm_pgtable.h | 4 ++++
>  arch/arm64/kvm/mmu.c                 | 2 +-
>  2 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index 19278dfe7978..b0c4651a4d9a 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -19,11 +19,15 @@
>   *  - 4K (level 1):	1GB
>   *  - 16K (level 2):	32MB
>   *  - 64K (level 2):	512MB
> + *
> + *  The max block level is the _smallest_ supported block size for KVM.

This feels like a non sequitur given the old comment is left in place...

>   */
>  #ifdef CONFIG_ARM64_4K_PAGES
>  #define KVM_PGTABLE_MIN_BLOCK_LEVEL	1
> +#define KVM_PGTABLE_MAX_BLOCK_LEVEL	2
>  #else
>  #define KVM_PGTABLE_MIN_BLOCK_LEVEL	2
> +#define KVM_PGTABLE_MAX_BLOCK_LEVEL	KVM_PGTABLE_MIN_BLOCK_LEVEL
>  #endif
>  
>  #define kvm_lpa2_is_enabled()		system_supports_lpa2()
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index dc04bc767865..1e927b306aee 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -41,7 +41,7 @@ static phys_addr_t __stage2_range_addr_end(phys_addr_t addr, phys_addr_t end,
>  
>  static phys_addr_t stage2_range_addr_end(phys_addr_t addr, phys_addr_t end)
>  {
> -	phys_addr_t size = kvm_granule_size(KVM_PGTABLE_MIN_BLOCK_LEVEL);
> +	phys_addr_t size = kvm_granule_size(KVM_PGTABLE_MAX_BLOCK_LEVEL);
>  
>  	return __stage2_range_addr_end(addr, end, size);
>  }

This doesn't feel right to me. A property that we had before is that
leaf entries are visited at most once, since every mapping size was
evenly divisible into KVM_PGTABLE_MIN_BLOCK_LEVEL.

Seems like we could wind up visiting a PUD mapping 512 times, at least
for 4K pages.

-- 
Thanks,
Oliver