Messages in this thread | | | Subject | Re: voluntary-preemption: understanding latency trace | From | Lee Revell <> | Date | Thu, 09 Sep 2004 17:02:18 -0400 |
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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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