On 10.01.24 11:48, Barry Song wrote:
On Wed, Jan 10, 2024 at 6:38 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
On 10/01/2024 10:30, Barry Song wrote:
On Wed, Jan 10, 2024 at 6:23 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
On 10/01/2024 09:09, Barry Song wrote:
On Wed, Jan 10, 2024 at 4:58 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
On 10/01/2024 08:02, Barry Song wrote:
On Wed, Jan 10, 2024 at 12:16 PM John Hubbard <jhubbard@xxxxxxxxxx> wrote:
On 1/9/24 19:51, Barry Song wrote:
On Wed, Jan 10, 2024 at 11:35 AM John Hubbard <jhubbard@xxxxxxxxxx> wrote:
...
Hi Ryan,
One thing that immediately came up during some recent testing of mTHP
on arm64: the pid requirement is sometimes a little awkward. I'm running
tests on a machine at a time for now, inside various containers and
such, and it would be nice if there were an easy way to get some numbers
for the mTHPs across the whole machine.
Just to confirm, you're expecting these "global" stats be truely global and not
per-container? (asking because you exploicitly mentioned being in a container).
If you want per-container, then you can probably just create the container in a
cgroup?
I'm not sure if that changes anything about thpmaps here. Probably
this is fine as-is. But I wanted to give some initial reactions from
just some quick runs: the global state would be convenient.
Thanks for taking this for a spin! Appreciate the feedback.
+1. but this seems to be impossible by scanning pagemap?
so may we add this statistics information in kernel just like
/proc/meminfo or a separate /proc/mthp_info?
Yes. From my perspective, it looks like the global stats are more useful
initially, and the more detailed per-pid or per-cgroup stats are the
next level of investigation. So feels odd to start with the more
detailed stats.
probably because this can be done without the modification of the kernel.
Yes indeed, as John said in an earlier thread, my previous attempts to add stats
directly in the kernel got pushback; DavidH was concerned that we don't really
know exectly how to account mTHPs yet
(whole/partial/aligned/unaligned/per-size/etc) so didn't want to end up adding
the wrong ABI and having to maintain it forever. There has also been some
pushback regarding adding more values to multi-value files in sysfs, so David
was suggesting coming up with a whole new scheme at some point (I know
/proc/meminfo isn't sysfs, but the equivalent files for NUMA nodes and cgroups
do live in sysfs).
Anyway, this script was my attempt to 1) provide a short term solution to the
"we need some stats" request and 2) provide a context in which to explore what
the right stats are - this script can evolve without the ABI problem.
The detailed per-pid or per-cgroup is still quite useful to my case in which
we set mTHP enabled/disabled and allowed sizes according to vma types,
eg. libc_malloc, java heaps etc.
Different vma types can have different anon_name. So I can use the detailed
info to find out if specific VMAs have gotten mTHP properly and how many
they have gotten.
However, Ryan did clearly say, above, "In future we may wish to
introduce stats directly into the kernel (e.g. smaps or similar)". And
earlier he ran into some pushback on trying to set up /proc or /sys
values because this is still such an early feature.
I wonder if we could put the global stats in debugfs for now? That's
specifically supposed to be a "we promise *not* to keep this ABI stable"
location.
Now that I think about it, I wonder if we can add a --global mode to the script
(or just infer global when neither --pid nor --cgroup are provided). I think I
should be able to determine all the physical memory ranges from /proc/iomem,
then grab all the info we need from /proc/kpageflags. We should then be able to
process it all in much the same way as for --pid/--cgroup and provide the same
stats, but it will apply globally. What do you think?
Having now thought about this for a few mins (in the shower, if anyone wants the
complete picture :) ), this won't quite work. This approach doesn't have the
virtual mapping information so the best it can do is tell us "how many of each
size of THP are allocated?" - it doesn't tell us anything about whether they are
fully or partially mapped or what their alignment is (all necessary if we want
to know if they are contpte-mapped). So I don't think this approach is going to
be particularly useful.
And this is also the big problem if we want to gather stats inside the kernel;
if we want something equivalant to /proc/meminfo's
AnonHugePages/ShmemPmdMapped/FilePmdMapped, we need to consider not just the
allocation of the THP but also whether it is mapped. That's easy for
PMD-mappings, because there is only one entry to consider - when you set it, you
increment the number of PMD-mapped THPs, when you clear it, you decrement. But
for PTE-mappings it's harder; you know the size when you are mapping so its easy
to increment, but you can do a partial unmap, so you would need to scan the PTEs
to figure out if we are unmapping the first page of a previously
fully-PTE-mapped THP, which is expensive. We would need a cheap mechanism to
determine "is this folio fully and contiguously mapped in at least one process?".
as OPPO's approach I shared to you before is maintaining two mapcount
1. entire map
2. subpage's map
3. if 1 and 2 both exist, it is DoubleMapped.
This isn't a problem for us. and everytime if we do a partial unmap,
we have an explicit
cont_pte split which will decrease the entire map and increase the
subpage's mapcount.
but its downside is that we expose this info to mm-core.
OK, but I think we have a slightly more generic situation going on with the
upstream; If I've understood correctly, you are using the PTE_CONT bit in the
PTE to determne if its fully mapped? That works for your case where you only
have 1 size of THP that you care about (contpte-size). But for the upstream, we
have multi-size THP so we can't use the PTE_CONT bit to determine if its fully
mapped because we can only use that bit if the THP is at least 64K and aligned,
and only on arm64. We would need a SW bit for this purpose, and the mm would
need to update that SW bit for every PTE one the full -> partial map transition.
My current implementation does use cont_pte but i don't think it is a must-have.
we don't need a bit in PTE to know if we are partially unmapping a large folio
at all.
as long as we are unmapping a part of a large folio, we do know what we are
doing. if a large folio is mapped entirely in a process, we get only
entire_map +1,
if we are unmapping a subpage of it, we get entire_map -1 and remained subpage's
mapcount + 1. if we are only mapping a part of this large folio, we
only increase
its subpages' mapcount.
That doesn't work as soon as you unmap a second subpage. Not to mention
that people ( :) ) are working on removing the subpage mapcounts.
I'm going propose that as a topic for LSF/MM soon, once I get to it.
--
Cheers,
David / dhildenb
