Message-ID: <CAGsJ_4wkQxAUxBtk2Wps_n07-3gSNrg0CkhL5RRDtu2E=Q+Wng@mail.gmail.com>
Date: Fri, 9 Aug 2024 19:37:36 +0800
From: Barry Song <baohua@...nel.org>
To: Ryan Roberts <ryan.roberts@....com>
Cc: David Hildenbrand <david@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Jonathan Corbet <corbet@....net>, Lance Yang <ioworker0@...il.com>, 
	Baolin Wang <baolin.wang@...ux.alibaba.com>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org
Subject: Re: [PATCH v2] mm: Override mTHP "enabled" defaults at kernel cmdline

On Fri, Aug 9, 2024 at 6:37 PM Ryan Roberts <ryan.roberts@....com> wrote:
>
> On 09/08/2024 10:32, David Hildenbrand wrote:
> > On 09.08.24 11:24, Barry Song wrote:
> >> On Fri, Aug 9, 2024 at 5:19 PM David Hildenbrand <david@...hat.com> wrote:
> >>>
> >>> On 08.08.24 12:16, Ryan Roberts wrote:
> >>>> Add thp_anon= cmdline parameter to allow specifying the default
> >>>> enablement of each supported anon THP size. The parameter accepts the
> >>>> following format and can be provided multiple times to configure each
> >>>> size:
> >>>>
> >>>> thp_anon=<size>[KMG]:<value>
> >>>>
> >>>> See Documentation/admin-guide/mm/transhuge.rst for more details.
> >>>>
> >>>> Configuring the defaults at boot time is useful to allow early user
> >>>> space to take advantage of mTHP before its been configured through
> >>>> sysfs.
> >>>
> >>> I suspect a khugeapged enhancement and/or kernel-config-dependant
> >>> defaults and/or early system settings will also be able to mitigate that
> >>> without getting kernel cmdlines involved in the future.
> >>>
> >>>>
> >>>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> >>>> ---
> >>>>
> >>>> Hi All,
> >>>>
> >>>> I've split this off from my RFC at [1] because Barry highlighted that he would
> >>>> benefit from it immediately [2]. There are no changes vs the version in that
> >>>> series.
> >>>>
> >>>> It applies against today's mm-unstable (275d686abcb59). (although I had to
> >>>> fix a
> >>>> minor build bug in stackdepot.c due to MIN() not being defined in this tree).
> >>>>
> >>>> Thanks,
> >>>> Ryan
> >>>>
> >>>>
> >>>>    .../admin-guide/kernel-parameters.txt         |  8 +++
> >>>>    Documentation/admin-guide/mm/transhuge.rst    | 26 +++++++--
> >>>>    mm/huge_memory.c                              | 55 ++++++++++++++++++-
> >>>>    3 files changed, 82 insertions(+), 7 deletions(-)
> >>>>
> >>>> diff --git a/Documentation/admin-guide/kernel-parameters.txt
> >>>> b/Documentation/admin-guide/kernel-parameters.txt
> >>>> index bcdee8984e1f0..5c79b58c108ec 100644
> >>>> --- a/Documentation/admin-guide/kernel-parameters.txt
> >>>> +++ b/Documentation/admin-guide/kernel-parameters.txt
> >>>> @@ -6631,6 +6631,14 @@
> >>>>                        <deci-seconds>: poll all this frequency
> >>>>                        0: no polling (default)
> >>>>
> >>>> +     thp_anon=       [KNL]
> >>>> +                     Format: <size>[KMG]:always|madvise|never|inherit
> >>>> +                     Can be used to control the default behavior of the
> >>>> +                     system with respect to anonymous transparent hugepages.
> >>>> +                     Can be used multiple times for multiple anon THP sizes.
> >>>> +                     See Documentation/admin-guide/mm/transhuge.rst for more
> >>>> +                     details.
> >>>> +
> >>>>        threadirqs      [KNL,EARLY]
> >>>>                        Force threading of all interrupt handlers except those
> >>>>                        marked explicitly IRQF_NO_THREAD.
> >>>> diff --git a/Documentation/admin-guide/mm/transhuge.rst
> >>>> b/Documentation/admin-guide/mm/transhuge.rst
> >>>> index 24eec1c03ad88..f63b0717366c6 100644
> >>>> --- a/Documentation/admin-guide/mm/transhuge.rst
> >>>> +++ b/Documentation/admin-guide/mm/transhuge.rst
> >>>> @@ -284,13 +284,27 @@ that THP is shared. Exceeding the number would block
> >>>> the collapse::
> >>>>
> >>>>    A higher value may increase memory footprint for some workloads.
> >>>>
> >>>> -Boot parameter
> >>>> -==============
> >>>> +Boot parameters
> >>>> +===============
> >>>>
> >>>> -You can change the sysfs boot time defaults of Transparent Hugepage
> >>>> -Support by passing the parameter ``transparent_hugepage=always`` or
> >>>> -``transparent_hugepage=madvise`` or ``transparent_hugepage=never``
> >>>> -to the kernel command line.
> >>>> +You can change the sysfs boot time default for the top-level "enabled"
> >>>> +control by passing the parameter ``transparent_hugepage=always`` or
> >>>> +``transparent_hugepage=madvise`` or ``transparent_hugepage=never`` to the
> >>>> +kernel command line.
> >>>> +
> >>>> +Alternatively, each supported anonymous THP size can be controlled by
> >>>> +passing ``thp_anon=<size>[KMG]:<state>``, where ``<size>`` is the THP size
> >>>> +and ``<state>`` is one of ``always``, ``madvise``, ``never`` or
> >>>> +``inherit``.
> >>>> +
> >>>> +For example, the following will set 64K THP to ``always``::
> >>>> +
> >>>> +     thp_anon=64K:always
> >>>> +
> >>>> +``thp_anon=`` may be specified multiple times to configure all THP sizes as
> >>>> +required. If ``thp_anon=`` is specified at least once, any anon THP sizes
> >>>> +not explicitly configured on the command line are implicitly set to
> >>>> +``never``.
> >>>
> >>> I suggest documenting that "thp_anon=" will not effect the value of
> >>> "transparent_hugepage=", or any configured default.
>
> Did you see the previous conversation with Barry about whether or not to honour
> configured defaults when any thp_anon= is provided [1]? Sounds like you also
> think we should honour the PMD "inherit" default if not explicitly provided on
> the command line? (see link for justification for the approach I'm currently
> taking).
>
> [1]
> https://lore.kernel.org/linux-mm/CAGsJ_4x8ruPspuk_FQVggJMWcXLbRuZFq44gg-Dt7Ewt3ExqTw@mail.gmail.com/
>
> >>>
> >>> Wondering if a syntax like
> >>>
> >>> thp_anon=16K,32K,64K:always;1048K,2048K:madvise
>
> Are there examples of that syntax already or have you just made it up? I found
> examples with the colon (:) but nothing this fancy. I guess that's not a reason
> not to do it though (other than the risk of screwing up the parser in a subtle way).
>
> >>>
> >>> (one could also support ranges, like "16K-64K")
> >>>
> >>> Would be even better. Then, maybe only allow a single instance.
> >>>
> >>> Maybe consider it if it's not too crazy to parse ;)
> I'll take a look. I'm going to be out for 3 weeks from end of Monday though, so
> probably won't get around to that until I'm back. I know Barry is keen to get
> this merged, so Barry, if you'd like to take it over that's fine by me (I'm sure
> you have enough on your plate though).

Yes, this command line is definitely needed. However, I also agree with
David that it might be worth considering options like '16K-64K' or '16K,
32K, 64K' after further reflection. For example, if users want to enable
six sizes, having six separate commands could end up being quite
silly :-)

I can address this after the mTHP number counters series.

>
> >>
> >> I prefer the current approach because it effectively filters cases like this.
> >>
> >> [    0.000000] huge_memory: thp_anon=8K:inherit: cannot parse, ignored
> >> [    0.000000] Unknown kernel command line parameters
> >> "thp_anon=8K:inherit", will be passed to user space.
> >>
> >> if we put multiple sizes together, 8K,32K,64K:always
> >>
> >> We can't determine whether this command line is legal or illegal as it
> >> is partially legal and partially illegal.
> >
> > Besides: I wouldn't bother about this "user does something stupid" scenario that
> > much.
> >
> > But yes, once we support more sizes a cmdline might turn invalid on an older
> > kernel.
> >
> > However, I don't see the problem here. User passed a non-existant size. Ignore
> > that one but handle the others, like you would with multiple commands?
>
> Yep, the parser could emit a warning for the size and move on.
>
> >
> > It can be well defined and documented. The command line is legal, just one size
> > does not exist.
> >
> > The world will continue turning :)

Right.

Thanks
Barry

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