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

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Message-ID: <CA+EESO4rMS5ot_wJfWVT+7rDKYgk187xnZJ3NNZsFgUXCbG1yQ@mail.gmail.com>
Date: Wed, 13 Aug 2025 15:01:59 -0700
From: Lokesh Gidra <lokeshgidra@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: aarcange@...hat.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	21cnbao@...il.com, ngeoffray@...gle.com, 
	Suren Baghdasaryan <surenb@...gle.com>, Kalesh Singh <kaleshsingh@...gle.com>, 
	Barry Song <v-songbaohua@...o.com>, David Hildenbrand <david@...hat.com>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v5] userfaultfd: opportunistic TLB-flush batching for
 present pages in MOVE

On Wed, Aug 13, 2025 at 2:47 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Wed, 13 Aug 2025 12:30:24 -0700 Lokesh Gidra <lokeshgidra@...gle.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?
No, these inefficiencies existed since MOVE ioctl came into existence.
IOW, the patch is improving performance, rather than fixing a
regression.