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SubjectRe: voluntary-preemption: understanding latency trace
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On Thu, 2004-09-09 at 06:41, Kevin Hilman wrote:
> I'm seeing a mismatch between my manually-measured timings and the
> timings I see in /proc/latency_trace.
>
> I've got a SCHED_FIFO kernel thread at the highest priority
> (MAX_USER_RT_PRIO-1) and it's sleeping on a wait queue. The wake is
> called from an ISR. Since this thread is the highest priority in the
> system, I expect it to run before the ISR threads and softIRQ threads
> etc.
>
> In the ISR I sample sched_clock() just before the call to wake_up()
> and in the thread I sample sched_clock() again just after the call to
> sleep. I'm seeing an almost 4ms latency between the call to wake_up
> and the actual wakeup. However, in /proc/latency_trace, the worst
> latency I see during the running of this test is <500us.
>
> I must be misunderstanding how the latency traces are
> started/stopped. Can anyone shed some light? Thanks.
>
> My current setup is using -R5, running on a PII 400MHz system.
>

Ingo, any ideas here? Looks like maybe the use of sched_clock is the
problem.

Lee

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