[Devel] Re: [RFC][-mm] [0/2] Basic stats for cgroups V2

[Paul Menage]

On Sat, Apr 5, 2008 at 11:09 AM, Balaji Rao <balajirrao at gmail.com> wrote:
> V1->V2
>  - Fixed a possible race in cpu_cgroup_read_stat. Thank you Paul for pointing this out.
>  - A few other naming changes.
>
>  This patchset is a first step towards implementing stats for cgroup
>  subsystems. Only a few trivial stats for cpu and memory resource controller
>  have been implemented for now. Please provide comments on the general
>  direction and any suggestions on how you would like the cgroupstats framework
>  to be implemented.

This is sort of heading in the same way as the cgroup binary stats API
that I mentioned a couple of months ago (when I proposed the
"cgroup.api" file).

Since the cgroup file API encourages subsystems to export values via
abstract methods such as read_s64() or read_map() rather than having
them handle the file I/O themselves, this gives the basis for a binary
stats API - the same methods can be used to retrieve the information
in a binary form rather than from regular ASCII-based file reads, and
the subsystem doesn't have to care which is being used.

I was originally thinking along the lines of having a special mode in
which you could obtain a cgroupfs binary file for a cgroup directory
that would report a requested set of binary stats each time it was
read, but using the netlink/taskstats API might be a good approach
too.

One of the important API choices would be whether the stats API was
fixed in header files shared with userspace, or whether it would be
possible for stats to be added and dynamically discovered/used by
userspace without needing fixed header file descriptions.

The difference would be a bit like the old sysctl API (where each
sysctl entry had to be enumerated in a header file) versus the newer
/proc/sys approach where numerical values aren't used and userspace
can determine which entries are supported at runtime, and even access
new previously-unknown entries.

Here's one possible way to do it:

With the taskstats interface, we could have operations to:

- describe the API exported by a given subsystem (automatically
generated, based on its registered control files and their access
methods)

- retrieve a specified set of stats in a binary format

So as a concrete example, with the memory, cpuacct and cpu subsystems
configured, the reported API might look something like (in pseudo-code
form)

0 : memory.usage_in_bytes : u64
1 : memory.limit_in_bytes : u64
2 : memory.failcnt : u64
3 : memory.stat : map
4 : cpuacct.usage : u64
5 : cpu.shares : u64
6 : cpu.rt_runtime_ms : s64
7 : cpu.stat : map

This list would be auto-generated by cgroups based on inspection of
the control files.

The user could then request stats 0, 3 and 7 for a cgroup to get the
memory.usage_in_bytes, memory.stat and cpu.stat statistics.

The stats could be returned in a binary format; the format for each
individual stat would depend on the type of that stat, and these could
be simply concatenated together.

A u64 or s64 stat would simply be a 64-bit value in the data stream

A map stat would be represented as a sequence of 64-bit values,
representing the values in the map. There would be no need to include
the size of the map or the key ordering in the binary format, since
userspace could determine that by reading the ASCII version of the map
control file once at startup.

So in the case of the request above for stats 0, 3 & 7, the binary
stats stream would be a sequence of 64-bit values consisting of:

<memory.usage>
<memory.stat.cache>
<memory.stat.rss>
<memory.stat.active>
<memory.stat.inactive>
<cpu.stat.utime>
<cpu.stat.stime>

If more stats were added to memory.stat or cpu.stat by a future
version of the code, then they would automatically appear; any that
userspace didn't understand it could ignore.

The userspace side of this could be handled by libcg.

Thoughts?

Paul
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