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    SubjectRe: [patch] Latency Tracer, voluntary-preempt-2.6.8-rc4-O6

    * Florian Schmidt <mista.tapas@gmx.net> wrote:

    > Hmm, should a new report only be generated when the newly measured
    > latency is really > than all other ones? I get the feeling that >=
    > seems to be enough. Here a dmesg extract:
    >
    > (swapper/1): new 6 us maximum-latency critical section.
    > (swapper/1): new 8 us maximum-latency critical section.
    > (swapper/1): new 9 us maximum-latency critical section.

    > Here are two reports with the same maximum-latency (31 us):
    >
    > (swapper/1): new 31 us maximum-latency critical section.
    > (swapper/1): new 31 us maximum-latency critical section.

    the latency tracer tracks latencies in cycle units - but they are
    displayed at microsecond accuracy - hence these 'equal' latencies.

    to jump-start all those smaller latencies you can do this:

    echo 100 > /proc/sys/kernel/preempt_max_latency
    echo 0 > /proc/sys/kernel/preempt_thresh

    this way the maximum-searching only starts at 100 usecs.

    > Btw: i do have some regular ca. 300 us latencies.. Here are some traces
    > (these happen with an average frequency of ca. 0.3hz):

    > (ksoftirqd/0/2): 307 us critical section violates 250 us threshold.
    > => started at: <___do_softirq+0x20/0x90>
    > => ended at: <cond_resched_softirq+0x59/0x70>

    this is too opaque - could you try -O7, enable tracing and save a
    /proc/latency_trace instance of such a latency? It looks like some sort
    of softirq latency - perhaps one particular driver's timer fn causes it
    - we'll be able to tell more from the trace.

    Ingo
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