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SubjectRe: [PATCH] KVM: x86/mmu: Update number of zapped pages even if page list is stable
On Mon, Nov 15, 2021 at 11:23 AM Sean Christopherson <seanjc@google.com> wrote:
>
> On Mon, Nov 15, 2021, David Matlack wrote:
> > On Thu, Nov 11, 2021 at 2:14 PM Sean Christopherson <seanjc@google.com> wrote:
> > >
> > > When zapping obsolete pages, update the running count of zapped pages
> > > regardless of whether or not the list has become unstable due to zapping
> > > a shadow page with its own child shadow pages. If the VM is backed by
> > > mostly 4kb pages, KVM can zap an absurd number of SPTEs without bumping
> > > the batch count and thus without yielding. In the worst case scenario,
> > > this can cause an RCU stall.
> > >
> > > rcu: INFO: rcu_sched self-detected stall on CPU
> > > rcu: 52-....: (20999 ticks this GP) idle=7be/1/0x4000000000000000
> > > softirq=15759/15759 fqs=5058
> > > (t=21016 jiffies g=66453 q=238577)
> > > NMI backtrace for cpu 52
> > > Call Trace:
> > > ...
> > > mark_page_accessed+0x266/0x2f0
> > > kvm_set_pfn_accessed+0x31/0x40
> > > handle_removed_tdp_mmu_page+0x259/0x2e0
> > > __handle_changed_spte+0x223/0x2c0
> > > handle_removed_tdp_mmu_page+0x1c1/0x2e0
> > > __handle_changed_spte+0x223/0x2c0
> > > handle_removed_tdp_mmu_page+0x1c1/0x2e0
> > > __handle_changed_spte+0x223/0x2c0
> > > zap_gfn_range+0x141/0x3b0
> > > kvm_tdp_mmu_zap_invalidated_roots+0xc8/0x130
> >
> > This is a useful patch but I don't see the connection with this stall.
> > The stall is detected in kvm_tdp_mmu_zap_invalidated_roots, which runs
> > after kvm_zap_obsolete_pages. How would rescheduling during
> > kvm_zap_obsolete_pages help?
>
> Ah shoot, I copy+pasted the wrong splat. The correct, revelant backtrace is:

Ok that makes more sense :). Also that was a soft lockup rather than
an RCU stall.
>
> mark_page_accessed+0x266/0x2e0
> kvm_set_pfn_accessed+0x31/0x40
> mmu_spte_clear_track_bits+0x136/0x1c0
> drop_spte+0x1a/0xc0
> mmu_page_zap_pte+0xef/0x120
> __kvm_mmu_prepare_zap_page+0x205/0x5e0
> kvm_mmu_zap_all_fast+0xd7/0x190
> kvm_mmu_invalidate_zap_pages_in_memslot+0xe/0x10
> kvm_page_track_flush_slot+0x5c/0x80
> kvm_arch_flush_shadow_memslot+0xe/0x10
> kvm_set_memslot+0x1a8/0x5d0
> __kvm_set_memory_region+0x337/0x590
> kvm_vm_ioctl+0xb08/0x1040

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