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

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Message-Id: <20250813144737.c3f388313fe13ff44856daf5@linux-foundation.org>
Date: Wed, 13 Aug 2025 14:47:37 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Lokesh Gidra <lokeshgidra@...gle.com>
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, 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?