Re: [PATCH v5] userfaultfd: opportunistic TLB-flush batching for present pages in MOVE

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 13 Aug 2025 12:30:24 -0700 Lokesh Gidra <lokeshgidra@google.com> wrote:

> MOVE ioctl's runtime is dominated by TLB-flush cost, which is required
> for moving present pages. Mitigate this cost by opportunistically
> batching present contiguous pages for TLB flushing.
> 
> Without batching, in our testing on an arm64 Android device with UFFD GC,
> which uses MOVE ioctl for compaction, we observed that out of the total
> time spent in move_pages_pte(), over 40% is in ptep_clear_flush(), and
> ~20% in vm_normal_folio().
> 
> With batching, the proportion of vm_normal_folio() increases to over
> 70% of move_pages_pte() without any changes to vm_normal_folio().
> Furthermore, time spent within move_pages_pte() is only ~20%, which
> includes TLB-flush overhead.
> 
> When the GC intensive benchmark, which was used to gather the above
> numbers, is run on cuttlefish (qemu android instance on x86_64), the
> completion time of the benchmark went down from ~45mins to ~20mins.
> 
> Furthermore, system_server, one of the most performance critical system
> processes on android, saw over 50% reduction in GC compaction time on an
> arm64 android device.

Were these inefficiencies a regression relative to an earlier kernel?