Messages in this thread | | | Subject | Re: voluntary-preemption: understanding latency trace | From | Lee Revell <> | Date | Fri, 10 Sep 2004 03:09:04 -0400 |
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On Fri, 2004-09-10 at 02:37, Ingo Molnar wrote: > * Lee Revell <rlrevell@joe-job.com> wrote: > > > > 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. > > > Ingo, any ideas here? Looks like maybe the use of sched_clock is the > > problem. > > sched_clock() is not 100% accurate (it takes a few shortcuts to avoid a > division) but it should be better than 90% so 4 msec measured means > there's likely some big delay. > > if the priority setup is indeed as described above then the RT task > should have run much faster. First i'd suggest to check whether it's not > console printing (printing of a stacktrace or a latency trace) that > slows things down. >
Ah, this is probably it, this is pretty close to the latency I get when /proc/latency_trace is updated, and this is also the one latency that doesn't show in the traces by design.
rlrevell@mindpipe:~$ ./amlat-rlr/amlat 599.895 MHz secondsPerTick=0.000000 ticksPerSecond=599894954.372806 599.895 MHz Using rtc interval of 1024 u=0 latency = 53 microseconds latency = 60 microseconds latency = 60 microseconds latency = 62 microseconds latency = 66 microseconds latency = 76 microseconds latency = 78 microseconds latency = 2548 microseconds
The last line is the latency trace being updated. If I turn off tracing or set preempt_max_latency to a high value then amlat doesn't show these.
Lee
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