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

[David Hildenbrand <david@redhat.com>]

On 17.07.23 15:20, Ryan Roberts wrote:
> On 17/07/2023 14:06, David Hildenbrand wrote:
>> 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.
> 
> ACK
> 
>>
>> For this case here, I'd suggest first adding the general infrastructure and then
>> adding tunables we want to have on top.
> 
> OK, so 1 patch for the main infrastructure, then a patch to disable for
> MADV_NOHUGEPAGE and friends, then a further patch to set flexthp_unhinted_max
> via a sysctl?

MADV_NOHUGEPAGE handling for me falls under the category "required for 
correctness to not break existing workloads" and has to be there initially.

Anything that is rather a performance tunable (e.g., a sysctl to 
optimize) can be added on top and discussed separately.

At least IMHO :)

> 
>>
>> I agree that toggling that at runtime (for example via sysfs as raised by me
>> previously) would be nicer.
> 
> OK, I clearly misunderstood, I thought you were requesting a boot parameter.

Oh, sorry about that. I wanted to actually express 
"/sys/kernel/mm/transparent_hugepage/" sysctls where we can toggle that 
later at runtime as well.

> What's the ABI compat guarrantee for sysctls? I assumed that for a boot
> parameter it would be easier to remove in future if we wanted, but for sysctl,
> its there forever?

sysctl are hard/impossible to remove, yes. So we better make sure what 
we add has clear semantics.

If we ever want some real auto-tunable mode (and can actually implement 
it without harming performance; and I am skeptical), we might want to 
allow for setting such a parameter to "auto", for example.

> 
> Also, how do you feel about the naming and behavior of the parameter?

Very good question. "flexthp_unhinted_max" naming is a bit suboptimal.

For example, I'm not so sure if we should expose the feature to user 
space as "flexthp" at all. I think we should find a clearer feature name 
to begin with.

... maybe we can initially get away with dropping that parameter and 
default to something reasonably small (i.e., 64k as you have above)?

/sys/kernel/mm/transparent_hugepage/enabled=never and simply not get any 
thp.

-- 
Cheers,

David / dhildenb