1. 06 May, 2020 1 commit
    • Michael Ellerman's avatar
      Merge the lockless page table walk rework into next · 1f12096a
      Michael Ellerman authored
      This merges the lockless page table walk rework series from Aneesh.
      Because it touches powerpc KVM code we are sharing it with the kvm-ppc
      tree in our topic/ppc-kvm branch.
      
      This is the cover letter from Aneesh:
      
      Avoid IPI while updating page table entries.
      
      Problem Summary:
      Slow termination of KVM guest with large guest RAM config due to a
      large number of IPIs that were caused by clearing level 1 PTE
      entries (THP) entries. This is shown in the stack trace below.
      
      - qemu-system-ppc  [kernel.vmlinux]            [k] smp_call_function_many
         - smp_call_function_many
            - 36.09% smp_call_function_many
                 serialize_against_pte_lookup
                 radix__pmdp_huge_get_and_clear
                 zap_huge_pmd
                 unmap_page_range
                 unmap_vmas
                 unmap_region
                 __do_munmap
                 __vm_munmap
                 sys_munmap
                system_call
                 __munmap
                 qemu_ram_munmap
                 qemu_anon_ram_free
                 reclaim_ramblock
                 call_rcu_thread
                 qemu_thread_start
                 start_thread
                 __clone
      
      Why we need to do IPI when clearing PMD entries:
      This was added as part of commit: 13bd817b ("powerpc/thp: Serialize pmd clear against a linux page table walk")
      
      serialize_against_pte_lookup makes sure that all parallel lockless
      page table walk completes before we convert a PMD pte entry to regular
      pmd entry. We end up doing that conversion in the below scenarios
      
      1) __split_huge_zero_page_pmd
      2) do_huge_pmd_wp_page_fallback
      3) MADV_DONTNEED running parallel to page faults.
      
      local_irq_disable and lockless page table walk:
      
      The lockless page table walk work with the assumption that we can
      dereference the page table contents without holding a lock. For this
      to work, we need to make sure we read the page table contents
      atomically and page table pages are not going to be freed/released
      while we are walking the table pages. We can achieve by using a rcu
      based freeing for page table pages or if the architecture implements
      broadcast tlbie, we can block the IPI as we walk the page table pages.
      
      To support both the above framework, lockless page table walk is done
      with irq disabled instead of rcu_read_lock()
      
      We do have two interface for lockless page table walk, gup fast and
      __find_linux_pte. This patch series makes __find_linux_pte table walk
      safe against the conversion of PMD PTE to regular PMD.
      
      gup fast:
      
      gup fast is already safe against THP split because kernel now
      differentiate between a pmd split and a compound page split. gup fast
      can run parallel to a pmd split and we prevent a parallel gup fast to
      a hugepage split, by freezing the page refcount and failing the
      speculative page ref increment.
      
      Similar to how gup is safe against parallel pmd split, this patch
      series updates the __find_linux_pte callers to be safe against a
      parallel pmd split. We do that by enforcing the following rules.
      
      1) Don't reload the pte value, because that can be updated in
         parallel.
      2) Code should be able to work with a stale PTE value and not the
         recent one. ie, the pte value that we are looking at may not be the
         latest value in the page table.
      3) Before looking at pte value check for _PAGE_PTE bit. We now do this
      as part of pte_present() check.
      
      Performance:
      
      This speeds up Qemu guest RAM del/unplug time as below
      128 core, 496GB guest:
      
      Without patch:
        munmap start: timer = 13162 ms, PID=7684
        munmap finish: timer = 95312 ms, PID=7684 - delta = 82150 ms
      
      With patch (upto removing IPI)
        munmap start: timer = 196449 ms, PID=6681
        munmap finish: timer = 196488 ms, PID=6681 - delta = 39ms
      
      With patch (with adding the tlb invalidate in pmdp_huge_get_and_clear_full)
        munmap start: timer = 196345 ms, PID=6879
        munmap finish: timer = 196714 ms, PID=6879 - delta = 369ms
      
      Link: https://lore.kernel.org/r/20200505071729.54912-1-aneesh.kumar@linux.ibm.com
      1f12096a
  2. 05 May, 2020 23 commits
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