On Wed, 2013-05-29 at 06:33 -0700, Paul E. McKenney wrote: > > Is there something a little smarter we can do? Could we use > > on_each_cpu_cond() with a function that checks if the CPU really is > > fully idle? > > One recent change that should help is making the _rcuidle variants of > the tracing functions callable from both idle and irq. To make the > on_each_cpu_cond() approach work, event tracing would need to switch > from RCU (which might be preemptible RCU) to RCU-sched (whose read-side > critical sections can pair with on_each_cpu(). I have to defer to Steven > on whether this is a good approach. >
Just to be clear, the issue is only with the function tracer. This has nothing to do with trace events, as we have the _rcuidle() variants to deal with that. We want the function tracer to trace pretty much everything it can, especially a complex system like RCU. Thus, I would say that the burden goes onto the tracing facility to solve this and not prevent tracing critical parts of RCU. As you stated with the problem of in_irq(), there's a point where we are in an interrupt but the in_irq() isn't set yet. And this even shows up in function tracing: <idle>-0 0d... 141.555326: function: smp_apic_timer_interrupt <idle>-0 0d... 141.555327: function: native_apic_mem_write <idle>-0 0d... 141.555327: function: exit_idle <idle>-0 0d... 141.555327: function: irq_enter <idle>-0 0d... 141.555327: function: rcu_irq_enter <idle>-0 0d... 141.555328: function: idle_cpu <idle>-0 0d.h. 141.555328: function: tick_check_idle <idle>-0 0d.h. 141.555328: function: tick_check_oneshot_broadcast <idle>-0 0d.h. 141.555328: function: ktime_get Notice that we traced smp_apic_timer_interrupt, native_apic_mem_write, exit_idle, irq_enter, and rcu_irq_enter, before rcu even was informed that we are coming out of idle. Then idle_cpu was also traced before the preempt_count was changed to notify that we are in an interrupt (the 'h' in "0d.h."). -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
