On Mon, Nov 15, 2021 at 11:23 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > On Mon, Nov 15, 2021, David Matlack wrote: > > On Thu, Nov 11, 2021 at 2:14 PM Sean Christopherson <seanjc@xxxxxxxxxx> 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
