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From:   David Matlack <dmatlack@google.com>
Date:   Mon, 15 Nov 2021 11:29:40 -0800
Message-ID: <CALzav=fKycSowAyaymt9a9hpffbWnFeXvACC5pE5-rMpx+4H4g@mail.gmail.com>
Subject: Re: [PATCH] KVM: x86/mmu: Update number of zapped pages even if page
 list is stable
To:     Sean Christopherson <seanjc@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, Ben Gardon <bgardon@google.com>
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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