voluntary-preemption: understanding latency trace

[Kevin Hilman]

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.

Kevin 
http://hilman.org/
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