* Re: [PATCH] KVM: arm/arm64: Skip updating page table entry if no change
2018-08-10 11:13 [PATCH] KVM: arm/arm64: Skip updating page table entry if no change Punit Agrawal
@ 2018-08-10 11:27 ` Suzuki K Poulose
2018-08-10 14:40 ` Marc Zyngier
2018-08-13 14:27 ` Christoffer Dall
2 siblings, 0 replies; 5+ messages in thread
From: Suzuki K Poulose @ 2018-08-10 11:27 UTC (permalink / raw)
To: Punit Agrawal, kvmarm
Cc: linux-arm-kernel, linux-kernel, Marc Zyngier, Christoffer Dall
On 08/10/2018 12:13 PM, Punit Agrawal wrote:
> Contention on updating a page table entry by a large number of vcpus
> can lead to duplicate work when handling stage 2 page faults. As the
> page table update follows the break-before-make requirement of the
> architecture, it can lead to repeated refaults due to clearing the
> entry and flushing the tlbs.
>
> This problem is more likely when -
>
> * there are large number of vcpus
> * the mapping is large block mapping
>
> such as when using PMD hugepages (512MB) with 64k pages.
>
> Fix this by skipping the page table update if there is no change in
> the entry being updated.
>
> Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
> Cc: Marc Zyngier <marc.zyngier@arm.com>
> Cc: Christoffer Dall <christoffer.dall@arm.com>
> Cc: Suzuki Poulose <suzuki.poulose@arm.com>
> --
> virt/kvm/arm/mmu.c | 16 ++++++++++++++++
> 1 file changed, 16 insertions(+)
>
> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> index 1d90d79706bd..a66a5441ca2f 100644
> --- a/virt/kvm/arm/mmu.c
> +++ b/virt/kvm/arm/mmu.c
> @@ -1027,6 +1027,18 @@ static int stage2_set_pmd_huge(struct kvm *kvm, struct kvm_mmu_memory_cache
> VM_BUG_ON(pmd_present(*pmd) && pmd_pfn(*pmd) != pmd_pfn(*new_pmd));
>
> old_pmd = *pmd;
> + /*
> + * Multiple vcpus faulting on the same PMD entry, can lead to
> + * them sequentially updating the PMD with the same
> + * value. Following the break-before-make (pmd_clear()
> + * followed by tlb_flush()) process can hinder forward
> + * progress due to refaults generated on missing translations.
> + *
> + * Skip updating the page table if the entry is unchanged.
> + */
> + if (pmd_val(old_pmd) == pmd_val(*new_pmd))
> + return 0;
> +
> if (pmd_present(old_pmd)) {
> pmd_clear(pmd);
> kvm_tlb_flush_vmid_ipa(kvm, addr);
> @@ -1101,6 +1113,10 @@ static int stage2_set_pte(struct kvm *kvm, struct kvm_mmu_memory_cache *cache,
>
> /* Create 2nd stage page table mapping - Level 3 */
> old_pte = *pte;
> + /* Skip page table update if there is no change */
> + if (pte_val(old_pte) == pte_val(*new_pte))
> + return 0;
> +
> if (pte_present(old_pte)) {
> kvm_set_pte(pte, __pte(0));
> kvm_tlb_flush_vmid_ipa(kvm, addr);
>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] KVM: arm/arm64: Skip updating page table entry if no change
2018-08-10 11:13 [PATCH] KVM: arm/arm64: Skip updating page table entry if no change Punit Agrawal
2018-08-10 11:27 ` Suzuki K Poulose
@ 2018-08-10 14:40 ` Marc Zyngier
2018-08-10 18:51 ` Punit Agrawal
2018-08-13 14:27 ` Christoffer Dall
2 siblings, 1 reply; 5+ messages in thread
From: Marc Zyngier @ 2018-08-10 14:40 UTC (permalink / raw)
To: Punit Agrawal
Cc: kvmarm, linux-arm-kernel, linux-kernel, Christoffer Dall, Suzuki Poulose
Hi Punit,
On Fri, 10 Aug 2018 12:13:00 +0100,
Punit Agrawal <punit.agrawal@arm.com> wrote:
>
> Contention on updating a page table entry by a large number of vcpus
> can lead to duplicate work when handling stage 2 page faults. As the
> page table update follows the break-before-make requirement of the
> architecture, it can lead to repeated refaults due to clearing the
> entry and flushing the tlbs.
>
> This problem is more likely when -
>
> * there are large number of vcpus
> * the mapping is large block mapping
>
> such as when using PMD hugepages (512MB) with 64k pages.
>
> Fix this by skipping the page table update if there is no change in
> the entry being updated.
>
> Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
> Cc: Marc Zyngier <marc.zyngier@arm.com>
> Cc: Christoffer Dall <christoffer.dall@arm.com>
> Cc: Suzuki Poulose <suzuki.poulose@arm.com>
This definitely deserves a Cc to stable, and a Fixes: tag.
> --
> Hi,
>
> This problem was reported by a user when testing PUD hugepages. During
> VM restore when all threads are running cpu intensive workload, the
> refauting was causing the VM to not make any forward progress.
>
> This patch fixes the problem for PMD and PTE page fault handling.
>
> Thanks,
> Punit
>
> Change-Id: I04c9aa8b9fbada47deb1a171c9959f400a0d2a21
> ---
> virt/kvm/arm/mmu.c | 16 ++++++++++++++++
> 1 file changed, 16 insertions(+)
>
> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> index 1d90d79706bd..a66a5441ca2f 100644
> --- a/virt/kvm/arm/mmu.c
> +++ b/virt/kvm/arm/mmu.c
> @@ -1027,6 +1027,18 @@ static int stage2_set_pmd_huge(struct kvm *kvm, struct kvm_mmu_memory_cache
> VM_BUG_ON(pmd_present(*pmd) && pmd_pfn(*pmd) != pmd_pfn(*new_pmd));
>
> old_pmd = *pmd;
> + /*
> + * Multiple vcpus faulting on the same PMD entry, can lead to
> + * them sequentially updating the PMD with the same
> + * value. Following the break-before-make (pmd_clear()
> + * followed by tlb_flush()) process can hinder forward
> + * progress due to refaults generated on missing translations.
> + *
> + * Skip updating the page table if the entry is unchanged.
> + */
> + if (pmd_val(old_pmd) == pmd_val(*new_pmd))
> + return 0;
Shouldn't you take this opportunity to also refactor it with the above
VM_BUG_ON and the below pmd_present? At the moment, we end-up testing
pmd_present twice, and your patch is awfully similar to the VM_BUG_ON
one.
> +
> if (pmd_present(old_pmd)) {
> pmd_clear(pmd);
> kvm_tlb_flush_vmid_ipa(kvm, addr);
> @@ -1101,6 +1113,10 @@ static int stage2_set_pte(struct kvm *kvm, struct kvm_mmu_memory_cache *cache,
>
> /* Create 2nd stage page table mapping - Level 3 */
> old_pte = *pte;
> + /* Skip page table update if there is no change */
> + if (pte_val(old_pte) == pte_val(*new_pte))
> + return 0;
> +
> if (pte_present(old_pte)) {
> kvm_set_pte(pte, __pte(0));
> kvm_tlb_flush_vmid_ipa(kvm, addr);
Thanks,
M.
--
Jazz is not dead, it just smell funny.
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] KVM: arm/arm64: Skip updating page table entry if no change
2018-08-10 14:40 ` Marc Zyngier
@ 2018-08-10 18:51 ` Punit Agrawal
0 siblings, 0 replies; 5+ messages in thread
From: Punit Agrawal @ 2018-08-10 18:51 UTC (permalink / raw)
To: Marc Zyngier; +Cc: kvmarm, linux-arm-kernel, linux-kernel
Marc Zyngier <marc.zyngier@arm.com> writes:
> Hi Punit,
>
> On Fri, 10 Aug 2018 12:13:00 +0100,
> Punit Agrawal <punit.agrawal@arm.com> wrote:
>>
>> Contention on updating a page table entry by a large number of vcpus
>> can lead to duplicate work when handling stage 2 page faults. As the
>> page table update follows the break-before-make requirement of the
>> architecture, it can lead to repeated refaults due to clearing the
>> entry and flushing the tlbs.
>>
>> This problem is more likely when -
>>
>> * there are large number of vcpus
>> * the mapping is large block mapping
>>
>> such as when using PMD hugepages (512MB) with 64k pages.
>>
>> Fix this by skipping the page table update if there is no change in
>> the entry being updated.
>>
>> Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>> Cc: Christoffer Dall <christoffer.dall@arm.com>
>> Cc: Suzuki Poulose <suzuki.poulose@arm.com>
>
> This definitely deserves a Cc to stable, and a Fixes: tag.
Agreed.
For PMD the issue exists since commit ad361f093c1e ("KVM: ARM: Support
hugetlbfs backed huge pages") when the file lived in arch/arm/kvm. (v3.12+)
For PTE the issue exists since commit d5d8184d35c9 ("KVM: ARM: Memory
virtualization setup"). (v3.8+)
I'll split the fix into two patches and add a cc to stable.
>> --
>> Hi,
>>
>> This problem was reported by a user when testing PUD hugepages. During
>> VM restore when all threads are running cpu intensive workload, the
>> refauting was causing the VM to not make any forward progress.
>>
>> This patch fixes the problem for PMD and PTE page fault handling.
>>
>> Thanks,
>> Punit
>>
>> Change-Id: I04c9aa8b9fbada47deb1a171c9959f400a0d2a21
Just noticed this. Looks like the commit hook has escaped it's worktree.
>> ---
>> virt/kvm/arm/mmu.c | 16 ++++++++++++++++
>> 1 file changed, 16 insertions(+)
>>
>> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
>> index 1d90d79706bd..a66a5441ca2f 100644
>> --- a/virt/kvm/arm/mmu.c
>> +++ b/virt/kvm/arm/mmu.c
>> @@ -1027,6 +1027,18 @@ static int stage2_set_pmd_huge(struct kvm *kvm, struct kvm_mmu_memory_cache
>> VM_BUG_ON(pmd_present(*pmd) && pmd_pfn(*pmd) != pmd_pfn(*new_pmd));
>>
>> old_pmd = *pmd;
>> + /*
>> + * Multiple vcpus faulting on the same PMD entry, can lead to
>> + * them sequentially updating the PMD with the same
>> + * value. Following the break-before-make (pmd_clear()
>> + * followed by tlb_flush()) process can hinder forward
>> + * progress due to refaults generated on missing translations.
>> + *
>> + * Skip updating the page table if the entry is unchanged.
>> + */
>> + if (pmd_val(old_pmd) == pmd_val(*new_pmd))
>> + return 0;
>
> Shouldn't you take this opportunity to also refactor it with the above
> VM_BUG_ON and the below pmd_present? At the moment, we end-up testing
> pmd_present twice, and your patch is awfully similar to the VM_BUG_ON
> one.
I went for the minimal change keeping a backport in mind.
The VM_BUG_ON() is enabled only when CONFIG_DEBUG_VM is selected and
checks for pfn ignoring the attributes, while this fix checks for the
entire entry. Maybe I'm missing something, but I can't see an obvious
way to combine the checks.
I've eliminated the duplicate pmd_present() check and will post the
updated patches.
Thanks,
Punit
>> +
>> if (pmd_present(old_pmd)) {
>> pmd_clear(pmd);
>> kvm_tlb_flush_vmid_ipa(kvm, addr);
>> @@ -1101,6 +1113,10 @@ static int stage2_set_pte(struct kvm *kvm, struct kvm_mmu_memory_cache *cache,
>>
>> /* Create 2nd stage page table mapping - Level 3 */
>> old_pte = *pte;
>> + /* Skip page table update if there is no change */
>> + if (pte_val(old_pte) == pte_val(*new_pte))
>> + return 0;
>> +
>> if (pte_present(old_pte)) {
>> kvm_set_pte(pte, __pte(0));
>> kvm_tlb_flush_vmid_ipa(kvm, addr);
>
> Thanks,
>
> M.
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] KVM: arm/arm64: Skip updating page table entry if no change
2018-08-10 11:13 [PATCH] KVM: arm/arm64: Skip updating page table entry if no change Punit Agrawal
2018-08-10 11:27 ` Suzuki K Poulose
2018-08-10 14:40 ` Marc Zyngier
@ 2018-08-13 14:27 ` Christoffer Dall
2 siblings, 0 replies; 5+ messages in thread
From: Christoffer Dall @ 2018-08-13 14:27 UTC (permalink / raw)
To: Punit Agrawal
Cc: kvmarm, linux-arm-kernel, linux-kernel, Marc Zyngier, Suzuki Poulose
On Fri, Aug 10, 2018 at 12:13:00PM +0100, Punit Agrawal wrote:
> Contention on updating a page table entry by a large number of vcpus
> can lead to duplicate work when handling stage 2 page faults. As the
> page table update follows the break-before-make requirement of the
> architecture, it can lead to repeated refaults due to clearing the
> entry and flushing the tlbs.
>
> This problem is more likely when -
>
> * there are large number of vcpus
> * the mapping is large block mapping
>
> such as when using PMD hugepages (512MB) with 64k pages.
>
> Fix this by skipping the page table update if there is no change in
> the entry being updated.
>
> Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
> Cc: Marc Zyngier <marc.zyngier@arm.com>
> Cc: Christoffer Dall <christoffer.dall@arm.com>
> Cc: Suzuki Poulose <suzuki.poulose@arm.com>
> --
> Hi,
>
> This problem was reported by a user when testing PUD hugepages. During
> VM restore when all threads are running cpu intensive workload, the
> refauting was causing the VM to not make any forward progress.
>
> This patch fixes the problem for PMD and PTE page fault handling.
>
> Thanks,
> Punit
>
> Change-Id: I04c9aa8b9fbada47deb1a171c9959f400a0d2a21
> ---
> virt/kvm/arm/mmu.c | 16 ++++++++++++++++
> 1 file changed, 16 insertions(+)
>
> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> index 1d90d79706bd..a66a5441ca2f 100644
> --- a/virt/kvm/arm/mmu.c
> +++ b/virt/kvm/arm/mmu.c
> @@ -1027,6 +1027,18 @@ static int stage2_set_pmd_huge(struct kvm *kvm, struct kvm_mmu_memory_cache
> VM_BUG_ON(pmd_present(*pmd) && pmd_pfn(*pmd) != pmd_pfn(*new_pmd));
>
> old_pmd = *pmd;
> + /*
> + * Multiple vcpus faulting on the same PMD entry, can lead to
> + * them sequentially updating the PMD with the same
> + * value. Following the break-before-make (pmd_clear()
> + * followed by tlb_flush()) process can hinder forward
> + * progress due to refaults generated on missing translations.
> + *
> + * Skip updating the page table if the entry is unchanged.
> + */
> + if (pmd_val(old_pmd) == pmd_val(*new_pmd))
> + return 0;
> +
> if (pmd_present(old_pmd)) {
> pmd_clear(pmd);
> kvm_tlb_flush_vmid_ipa(kvm, addr);
> @@ -1101,6 +1113,10 @@ static int stage2_set_pte(struct kvm *kvm, struct kvm_mmu_memory_cache *cache,
>
> /* Create 2nd stage page table mapping - Level 3 */
> old_pte = *pte;
> + /* Skip page table update if there is no change */
> + if (pte_val(old_pte) == pte_val(*new_pte))
> + return 0;
> +
> if (pte_present(old_pte)) {
> kvm_set_pte(pte, __pte(0));
> kvm_tlb_flush_vmid_ipa(kvm, addr);
> --
> 2.18.0
>
Acked-by: Christoffer Dall <christoffer.dall@arm.com>
^ permalink raw reply [flat|nested] 5+ messages in thread