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* Process memory accounting (cgroups) accuracy
@ 2021-07-02  7:50 Krzysztof Kozlowski
  2021-07-02  9:08 ` Michal Hocko
  0 siblings, 1 reply; 3+ messages in thread
From: Krzysztof Kozlowski @ 2021-07-02  7:50 UTC (permalink / raw)
  To: Andrew Morton, Johannes Weiner, Michal Hocko, Vladimir Davydov,
	linux-mm, linux-kernel, cgroups


Since some time I am trying to fix Linux Test Project tests around
memory cgroups:

The trouble I have, for example with is
that on recent kernels (v4.15+) on x86_64, the memory group reports max
usage as higher than process' anonymous mapping.

The test works like this:
1. Fork a process, signal it to mmap 4 MB (PROT_WRITE | PROT_READ,
AP_PRIVATE | MAP_ANONYMOUS) and touch the memory.
2. Add the process to control group.
3. Signal it to munmap the region and immediately mmap again the same 4
MB (with touching the memory).
4. Check the counters and reset them.
5. munmap
6. Check the counters

Mentioned checks the counters of
memory.memsw.max_usage_in_bytes which are:
a. early kernels: 4 MB (so only the mmap)
b. v4.15, v5.4 kernel: 4 MB + 32 pages
c. v5.11 kernel: 4 MB + 32 pages + 2 pages

I tweaked the mmap() size to smaller values and then the accounting is
even different. For example mmap of 1 up to 32 pages the
memory.memsw.max_usage_in_bytes is always 131072.

After final munmap (point 5 above), the test expects the
memcg_max_usage_in_bytes to be =0, however it is usually 8 or 132 kB.
Which kind of points that process is charged for something not related
to that memory map directly.

The questions: How accurate are now the cgroup counters?
I understood they should charge only pages allocated by the process, so
why mmap(4 kB) causes max_usage_in_bytes=132 kB?
Why mmap(4 MB) causes max_usage_in_bytes=4 MB + 34 pages?
What is being accounted there (stack guards?)?

Or maybe the entire LTP test checking so carefully memcg limits is useless?

The v5.4 kernel config is here:

Best regards,

^ permalink raw reply	[flat|nested] 3+ messages in thread

end of thread, other threads:[~2021-07-02 10:40 UTC | newest]

Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
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2021-07-02  7:50 Process memory accounting (cgroups) accuracy Krzysztof Kozlowski
2021-07-02  9:08 ` Michal Hocko
2021-07-02 10:40   ` Krzysztof Kozlowski

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