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    SubjectRe: [RFC][PATCH v2 4/7] taskstats: Add per task steal time accounting
    On Mon, 15 Nov 2010 19:08:44 +0100
    Peter Zijlstra <> wrote:

    > On Mon, 2010-11-15 at 18:59 +0100, Martin Schwidefsky wrote:
    > > Steal time per task is at least good for performance problem analysis.
    > > Sometimes knowing what is not the cause of a performance problem can help you
    > > tremendously. If a task is slow and has no steal time, well then the hypervisor
    > > is likely not the culprit. On the other hand if you do see lots of steal time
    > > for a task while the rest of the system doesn't cause any steal time can tell
    > > you something as well. That task might hit a specific function which causes
    > > hypervisor overhead. The usefulness depends on the situation, it is another
    > > data point which may or may not help you.
    > If performance analysis is the only reason, why not add a tracepoint on
    > vcpu enter that reports the duration the vcpu was out for and use perf
    > to gather said data? It can tell you what process was running and what
    > instruction it was at when the vcpu went away.
    > No need to add 40 bytes per task for that.

    Which vcpu enter? We usually have z/VM as our hypervisor and want to be able
    to do performance analysis with the data we gather inside the guest. There
    is no vcpu enter we could put a tracepoint on. We would have to put tracepoints
    on every possible interaction point with z/VM to get this data. To me it seems
    a lot simpler to add the per-task steal time.

    And if it is really the additional 40 bytes on x86 that bother you so much,
    we could put them behind #ifdef CONFIG_VIRT_CPU_ACCOUNTING. There already
    is one in the task_struct for prev_utime and prev_stime.

    blue skies,

    "Reality continues to ruin my life." - Calvin.

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