On 14.07.23 19:17, Yu Zhao wrote:

On Fri, Jul 14, 2023 at 10:17 AM Ryan Roberts [off-list ref] wrote:

quoted

Introduce FLEXIBLE_THP feature, which allows anonymous memory to be
allocated in large folios of a determined order. All pages of the large
folio are pte-mapped during the same page fault, significantly reducing
the number of page faults. The number of per-page operations (e.g. ref
counting, rmap management lru list management) are also significantly
reduced since those ops now become per-folio.

The new behaviour is hidden behind the new FLEXIBLE_THP Kconfig, which
defaults to disabled for now; The long term aim is for this to defaut to
enabled, but there are some risks around internal fragmentation that
need to be better understood first.

When enabled, the folio order is determined as such: For a vma, process
or system that has explicitly disabled THP, we continue to allocate
order-0. THP is most likely disabled to avoid any possible internal
fragmentation so we honour that request.

Otherwise, the return value of arch_wants_pte_order() is used. For vmas
that have not explicitly opted-in to use transparent hugepages (e.g.
where thp=madvise and the vma does not have MADV_HUGEPAGE), then
arch_wants_pte_order() is limited by the new cmdline parameter,
`flexthp_unhinted_max`. This allows for a performance boost without
requiring any explicit opt-in from the workload while allowing the
sysadmin to tune between performance and internal fragmentation.

arch_wants_pte_order() can be overridden by the architecture if desired.
Some architectures (e.g. arm64) can coalsece TLB entries if a contiguous
set of ptes map physically contigious, naturally aligned memory, so this
mechanism allows the architecture to optimize as required.

If the preferred order can't be used (e.g. because the folio would
breach the bounds of the vma, or because ptes in the region are already
mapped) then we fall back to a suitable lower order; first
PAGE_ALLOC_COSTLY_ORDER, then order-0.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
  .../admin-guide/kernel-parameters.txt         |  10 +
  mm/Kconfig                                    |  10 +
  mm/memory.c                                   | 187 ++++++++++++++++--
  3 files changed, 190 insertions(+), 17 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a1457995fd41..405d624e2191 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt

@@ -1497,6 +1497,16 @@
                         See Documentation/admin-guide/sysctl/net.rst for
                         fb_tunnels_only_for_init_ns

+       flexthp_unhinted_max=
+                       [KNL] Requires CONFIG_FLEXIBLE_THP enabled. The maximum
+                       folio size that will be allocated for an anonymous vma
+                       that has neither explicitly opted in nor out of using
+                       transparent hugepages. The size must be a power-of-2 in
+                       the range [PAGE_SIZE, PMD_SIZE). A larger size improves
+                       performance by reducing page faults, while a smaller
+                       size reduces internal fragmentation. Default: max(64K,
+                       PAGE_SIZE). Format: size[KMG].
+

Let's split this parameter into a separate patch.

Just a general comment after stumbling over patch #2, let's not start 
splitting patches into things that don't make any sense on their own; 
that just makes review a lot harder.

For this case here, I'd suggest first adding the general infrastructure 
and then adding tunables we want to have on top.

I agree that toggling that at runtime (for example via sysfs as raised 
by me previously) would be nicer.

-- 
Cheers,

David / dhildenb


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