Messages in this thread | | | Date | Sat, 14 Aug 2004 17:52:19 +0100 | From | James Courtier-Dutton <> | Subject | Re: [patch] Latency Tracer, voluntary-preempt-2.6.8-rc4-O6 |
| |
Ingo Molnar wrote: > * James Courtier-Dutton <James@superbug.demon.co.uk> wrote: > > >>This is just for info, now that we have a nice latency testing tool, >>we might as well collect some useful traces that we can later work on. >> >>Here is a trace showing a latency of 39034us. >>http://www.superbug.demon.co.uk/kernel/ > > >>This looks to me to be a bug somewhere. Either in the O7 patch, or in >>the kernel. Surely, do_IRQ should happen quickly, and not take 39ms. > > > something's wrong indeed. Is this an x86 SMP system? If it's SMP then > please apply the patch below (ontop of -O7), it fixes an SMP false > positive bug in the latency timing code. > > the process is looping somewhere. Here are the non-IRQ trace entries: > > 0.001ms (+0.000ms): __switch_to (schedule) > 0.002ms (+0.000ms): finish_task_switch (schedule) > 0.002ms (+0.000ms): __preempt_spin_lock (schedule) > ... [lots of IRQs] ... > 38.126ms (+0.362ms): preempt_schedule (schedule) > 38.126ms (+0.000ms): sched_clock (schedule) > 38.127ms (+0.000ms): find_next_bit (schedule) > 38.127ms (+0.000ms): task_timeslice (schedule) > > this shows that we are looping in __preempt_spin_lock() - most likely > via schedule()'s reqacquire_kernel_lock() code. > > i.e. another CPU is holding the big kernel lock and this CPU is looping. > _but_ this CPU is fully preemptible so the trace produces this false > positive. > > Ingo >
I have not seen that particular problem any more, so I think the patch helped.
I have found a new problem though:
# cat latency_trace preemption latency trace v1.0 ----------------------------- latency: 1883455195 us, entries: 1 (1) process: ksoftirqd/1/5, uid: 0 nice: -10, policy: 0, rt_priority: 0 =======> 0.000ms (+0.000ms): cond_resched_softirq (___do_softirq)
That looks bad to me. The user did not notice any latency, but 1883 seconds seems like a high latency to me!
James - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |