Re: [PATCH v3 3/4] mm: FLEXIBLE_THP for improved performance

[David Hildenbrand <david@redhat.com>]

On 14.07.23 19:17, Yu Zhao wrote:
> On Fri, Jul 14, 2023 at 10:17 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> 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