Hello,
we use the seL4 benchmark interface to feed Genode's trace infrastructure with information about the CPU utilization of threads and of idle times.
In general the integration was reasonable straightforward.
One minor point we had. It seems that the idle time of a CPU can only be requested by a thread running on the vary same CPU. Requesting the CPU idle times of a remote CPU (the calling thread is on another CPU) is not supported, right ?
In principle we may start on each CPU a thread just for the sake of requesting the idle utilization time, which however looks a bit of overkill.
We "kind of" circumvent the issue for us by changing the syscall handling in the kernel in that regard, that the CPU number of the requested target thread is taken instead that of the caller thread, e.g. like this:
--- a/src/benchmark/benchmark_utilisation.c +++ b/src/benchmark/benchmark_utilisation.c @@ -36,6 +36,11 @@ void benchmark_track_utilisation_dump(void)
tcb = TCB_PTR(cap_thread_cap_get_capTCBPtr(lu_ret.cap)); buffer[BENCHMARK_TCB_UTILISATION] = tcb->benchmark.utilisation; /* Requested thread utilisation */ +#if CONFIG_MAX_NUM_NODES > 1 + if (tcb->tcbAffinity < ksNumCPUs) + buffer[BENCHMARK_IDLE_UTILISATION] = NODE_STATE_ON_CORE(ksIdleThread, tcb->tcbAffinity)->benchmark.utilisation; /* Idle thread utilisation */ + else +#endif buffer[BENCHMARK_IDLE_UTILISATION] = NODE_STATE(ksIdleThread)->benchmark.utilisation; /* Idle thread utilisation */
#ifdef CONFIG_ARM_ENABLE_PMU_OVERFLOW_INTERRUPT
With this change, we still have to create a thread on every CPU (since we need a capability for the syscall), but at least the threads must not be running actively.
Does this change (the patch) make sense to you and is it worthwhile to adjust in general on your side - or would you advise/envision another approach/direction ?
(like specifying the cpu number for the idle times directly via the syscall or having a explicit syscall just for cpu idle times [but there are no specific thread idle capabilities as syscall parameter] etc.)
Another question:
Currently it seems that no idle CPU times are provided, as long as no user thread is actively running on that specific CPU. We can handle it, no problem in general - however we would have to adjust generic code (which runs on 8 different kernels) specifically for seL4 to handle this minor case. Could this possibly be changed (easily) ?
Thanks,
Alex.