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    SubjectRe: [RFC][-mm] [0/2] Basic stats for cgroups V2
    On Sat, Apr 5, 2008 at 11:09 AM, Balaji Rao <balajirrao@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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