From: Christoffer Dall <christoffer.dall@arm.com>
To: Punit Agrawal <punit.agrawal@arm.com>
Cc: kvmarm@lists.cs.columbia.edu,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org, Marc Zyngier <marc.zyngier@arm.com>,
Suzuki Poulose <suzuki.poulose@arm.com>
Subject: Re: [PATCH] KVM: arm/arm64: Skip updating page table entry if no change
Date: Mon, 13 Aug 2018 16:27:11 +0200 [thread overview]
Message-ID: <20180813142711.GJ20606@e113682-lin.lund.arm.com> (raw)
In-Reply-To: <20180810111300.17144-1-punit.agrawal@arm.com>
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>
prev parent reply other threads:[~2018-08-13 14:27 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
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 [this message]
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