linux-kernel - Re: [PATCH v3] userfaultfd: opportunistic TLB-flush batching for present pages in MOVE

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Message-ID: <CA+EESO7vtVBUdppuUVTazaK0Aaxy6=FCWX-7YrWvg1Y93oqdOw@mail.gmail.com>
Date: Tue, 12 Aug 2025 07:01:59 -0700
From: Lokesh Gidra <lokeshgidra@...gle.com>
To: Peter Xu <peterx@...hat.com>
Cc: akpm@...ux-foundation.org, 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>
Subject: Re: [PATCH v3] userfaultfd: opportunistic TLB-flush batching for
 present pages in MOVE

On Mon, Aug 11, 2025 at 7:00 AM Peter Xu <peterx@...hat.com> wrote:
>
> On Fri, Aug 08, 2025 at 09:29:58AM -0700, Lokesh Gidra wrote:
> > On Thu, Aug 7, 2025 at 12:17 PM Peter Xu <peterx@...hat.com> wrote:
> > >
> > > Hi, Lokesh,
> > >
> > > On Thu, Aug 07, 2025 at 03:39:02AM -0700, Lokesh Gidra 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().
> > >
> > > Do you know why vm_normal_folio() could be expensive? I still see quite
> > > some other things this path needs to do.
> > >
> > Let's discuss this in Andrew's reply thread.
>
> Sorry to get back to this late.  Thanks for the link, Andrew!
>
> >
> > > > Furthermore, time spent within move_pages_pte() is only ~20%, which
> > > > includes TLB-flush overhead.
> > >
> > > Indeed this should already prove the optimization, I'm just curious whether
> > > you've run some benchmark on the GC app to show the real world benefit.
> > >
> > I did! The same benchmark through which I gathered these numbers, when
> > run on cuttlefish (qemu android instance on x86_64), the completion
> > time of the benchmark went down from ~45mins to ~20mins. The benchmark
> > is very GC intensive and the overhead of IPI on vCPUs seems to be
> > enormous leading to this drastic improvement.
> >
> > In another instance, system_server, one of the most critical system
> > processes on android, saw over 50% reduction in GC compaction time on
> > an arm64 android device.
>
> Would you mind add some of these numbers into the commit message when you
> repost?

I can certainly do that in v5. But I already sent v4 addressing all
your other feedback. I'll incorporate these numbers as well as any
additional changes suggested in v4 in v5.
>
> Thanks,
>
> --
> Peter Xu
>