Re: [PATCH v2] mm: Override mTHP "enabled" defaults at kernel cmdline

[David Hildenbrand <david@redhat.com>]

>>>>> -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).

I primarily think that we should document it :)

What if someone passes "transparent_hugepage=always" and "thp_anon=..."? 
I would assume that transparent_hugepage would only affect the global 
toggle then?

> 
> [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).

I made it up -- mostly ;) I think we are quite flexible on what we can 
do. As always, maybe we can keep it bit consistent with existing stuff.

For hugetlb_cma we have things like
	"<node>:nn[KMGTPE|[,<node>:nn[KMGTPE]]

"memmap=" options are more ... advanced, including memory ranges. There 
are a bunch more documented in kernel-parameters.txt that have more 
elaborate formats.

Ranges would probably be the most valuable addition. So maybe we should 
start with:

	thp_anon=16K-64K:always,1048K-2048K:madvise

So to enable all THPs it would simply be

	thp_anon=16K-2M:always

Interesting question what would happen if someone passes:

	thp_anon=8K-2M:always

Likely we simply would apply it to any size in the range, even if 
start/end is not a THP size.

But we would want to complain to the user if someone only specifies a 
single one (or a range does not even select a single one) that does not 
exist:

	thp_anon=8K:always

> 
>>>>
>>>> (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

Oh, lucky you! Enjoy!

> 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).


-- 
Cheers,

David / dhildenb