-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@xxxxxxxxxxxxxx/
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
