From: Sean Christopherson <seanjc@google.com>
To: David Matlack <dmatlack@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>
Subject: Re: [PATCH] KVM: x86/mmu: Update number of zapped pages even if page list is stable
Date: Mon, 15 Nov 2021 19:23:27 +0000 [thread overview]
Message-ID: <YZKzr4mn1jJ3vdqK@google.com> (raw)
In-Reply-To: <CALzav=dpzzKgaNRLrSBy71WBvybWmRJ39eDv4hPXsbU_DSS-fA@mail.gmail.com>
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:
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
next prev parent reply other threads:[~2021-11-15 20:45 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-11 22:14 [PATCH] KVM: x86/mmu: Update number of zapped pages even if page list is stable Sean Christopherson
2021-11-11 22:31 ` Ben Gardon
2021-11-11 23:18 ` Sean Christopherson
2021-11-15 19:20 ` David Matlack
2021-11-15 19:23 ` Sean Christopherson [this message]
2021-11-15 19:29 ` David Matlack
2021-11-15 21:46 ` Sean Christopherson
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