(2013/12/07 10:32), Frank Ch. Eigler wrote: > Hi - > > On Sat, Dec 07, 2013 at 08:19:13AM +0900, Masami Hiramatsu wrote: > >> [...] >>> Would you plan to limit kprobes (or just the perf-probe frontend) to >>> only function-entries also? > >> Exactly, yes :). Currently I have a patch for kprobe-tracer >> implementation (not only for perf-probe, but doesn't limit kprobes >> itself). > > Interesting option. It sounds like a restrictive expedient that could > result in kprobes never being made sufficiently robust.
the raw-kprobes users like systemtap can also implement its own whitelist. :) ftrace-based whitelist is only useful for ftrace/perf. Anyway, the list is open via debugfs as available_filter_functions. >>> If not, and if intra-function statement-granularity kprobes remain >>> allowed within a function-granularity whitelist, then you might >>> still have those "quantitative" problems. > >> Yes, but as far as I've tested, the performance overhead is not >> high, especially as far as putting kprobes at the entry of those >> functions because of ftrace-based optimization. > > (Would that also make CONFIG_KPROBE_EVENT require KPROBES_ON_FTRACE?) Ah, no but a good point. at least the whitelist requires CONFIG_FUNCTION_TRACER. >>> Even worse, kprobes robustness problems can bite even with a small >>> whitelist, unless you can test the countless subset selections >>> cartesian-product the aggrevating factors (like other tracing >>> facilities being in use at the same time, limited memory, high irq >>> rates, debugging sessions, architectures, whatever). >> >> And also, what script will run on each probe, right? :) > > In the perf-probe world, the closest analogue could be varying the > contextual data that's being extracted (stack traces, parameters, ...). Yes, it should be verified before accessing it (and already done). >>>> [...] For the long term solution, I think we can introduce some >>>> kind of performance gatekeeper as systemtap does. Counting the >>>> miss-hit rate per second and if it go over a threshold, disable next >>>> miss-hit (or most miss-hit) probe (as OOM killer does). >>> >>> That would make sense, but again it would not help deal with kprobes >>> robustness (in the kernel-crashing rather than kernel-slowdown sense). >> >> Why would you think so? Is there any hidden path for calling kprobes >> mechanism?? The kernel crash problem just comes from bugs, not the >> quantitative issue. > > I don't think we're disagreeing. A performance-gatekeeper in > perf-probe or nearby would be useful (and manage the kprobe-quantity > problem). It would not be sufficient to prevent the kernel-crashing > bugs. Right. Ah, I just meant that we'd better add those features, not replacing the blacklist. And the blacklist should be maintained anyway. :) Thank you, -- Masami HIRAMATSU IT Management Research Dept. Linux Technology Center Hitachi, Ltd., Yokohama Research Laboratory E-mail: masami.hiramatsu...@hitachi.com -- 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/
