Messages in this thread | | | From | Andy Lutomirski <> | Subject | Re: [patch 4/8] x86/entry: Move irq tracing on syscall entry to C-code | Date | Sun, 1 Mar 2020 11:39:42 -0800 |
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> On Mar 1, 2020, at 11:30 AM, Paul E. McKenney <paulmck@kernel.org> wrote: > > On Sun, Mar 01, 2020 at 10:54:23AM -0800, Andy Lutomirski wrote: >>> On Sun, Mar 1, 2020 at 10:26 AM Paul E. McKenney <paulmck@kernel.org> wrote: >>> >>> On Sun, Mar 01, 2020 at 07:12:25PM +0100, Thomas Gleixner wrote: >>>> Andy Lutomirski <luto@kernel.org> writes: >>>>> On Sun, Mar 1, 2020 at 7:21 AM Thomas Gleixner <tglx@linutronix.de> wrote: >>>>>> Andy Lutomirski <luto@amacapital.net> writes: >>>>>>>> On Mar 1, 2020, at 2:16 AM, Thomas Gleixner <tglx@linutronix.de> wrote: >>>>>>>> Ok, but for the time being anything before/after CONTEXT_KERNEL is unsafe >>>>>>>> except trace_hardirq_off/on() as those trace functions do not allow to >>>>>>>> attach anything AFAICT. >>>>>>> >>>>>>> Can you point to whatever makes those particular functions special? I >>>>>>> failed to follow the macro maze. >>>>>> >>>>>> Those are not tracepoints and not going through the macro maze. See >>>>>> kernel/trace/trace_preemptirq.c >>>>> >>>>> That has: >>>>> >>>>> void trace_hardirqs_on(void) >>>>> { >>>>> if (this_cpu_read(tracing_irq_cpu)) { >>>>> if (!in_nmi()) >>>>> trace_irq_enable_rcuidle(CALLER_ADDR0, CALLER_ADDR1); >>>>> tracer_hardirqs_on(CALLER_ADDR0, CALLER_ADDR1); >>>>> this_cpu_write(tracing_irq_cpu, 0); >>>>> } >>>>> >>>>> lockdep_hardirqs_on(CALLER_ADDR0); >>>>> } >>>>> EXPORT_SYMBOL(trace_hardirqs_on); >>>>> NOKPROBE_SYMBOL(trace_hardirqs_on); >>>>> >>>>> But this calls trace_irq_enable_rcuidle(), and that's the part of the >>>>> macro maze I got lost in. I found: >>>>> >>>>> #ifdef CONFIG_TRACE_IRQFLAGS >>>>> DEFINE_EVENT(preemptirq_template, irq_disable, >>>>> TP_PROTO(unsigned long ip, unsigned long parent_ip), >>>>> TP_ARGS(ip, parent_ip)); >>>>> >>>>> DEFINE_EVENT(preemptirq_template, irq_enable, >>>>> TP_PROTO(unsigned long ip, unsigned long parent_ip), >>>>> TP_ARGS(ip, parent_ip)); >>>>> #else >>>>> #define trace_irq_enable(...) >>>>> #define trace_irq_disable(...) >>>>> #define trace_irq_enable_rcuidle(...) >>>>> #define trace_irq_disable_rcuidle(...) >>>>> #endif >>>>> >>>>> But the DEFINE_EVENT doesn't have the "_rcuidle" part. And that's >>>>> where I got lost in the macro maze. I looked at the gcc asm output, >>>>> and there is, indeed: >>>> >>>> DEFINE_EVENT >>>> DECLARE_TRACE >>>> __DECLARE_TRACE >>>> __DECLARE_TRACE_RCU >>>> static inline void trace_##name##_rcuidle(proto) >>>> __DO_TRACE >>>> if (rcuidle) >>>> .... >>>> >>>>> But I also don't see why this is any different from any other tracepoint. >>>> >>>> Indeed. I took a wrong turn at some point in the macro jungle :) >>>> >>>> So tracing itself is fine, but then if you have probes or bpf programs >>>> attached to a tracepoint these use rcu_read_lock()/unlock() which is >>>> obviosly wrong in rcuidle context. >>> >>> Definitely, any such code needs to use tricks similar to that of the >>> tracing code. Or instead use something like SRCU, which is OK with >>> readers from idle. Or use something like Steve Rostedt's workqueue-based >>> approach, though please be very careful with this latter, lest the >>> battery-powered embedded guys come after you for waking up idle CPUs >>> too often. ;-) >> >> Are we okay if we somehow ensure that all the entry code before >> enter_from_user_mode() only does rcuidle tracing variants and has >> kprobes off? Including for BPF use cases? > > That would work, though if BPF used SRCU instead of RCU, this would > be unnecessary. Sadly, SRCU has full memory barriers in each of > srcu_read_lock() and srcu_read_unlock(), but we are working on it. > (As always, no promises!) > >> It would be *really* nice if we could statically verify this, as has >> been mentioned elsewhere in the thread. It would also probably be >> good enough if we could do it at runtime. Maybe with lockdep on, we >> verify rcu state in tracepoints even if the tracepoint isn't active? >> And we could plausibly have some widget that could inject something >> into *every* kprobeable function to check rcu state. > > Or just have at least one testing step that activates all tracepoints, > but with lockdep enabled?
Also kprobe.
I don’t suppose we could make notrace imply nokprobe. Then all kprobeable functions would also have entry/exit tracepoints, right?
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