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Re: [PATCH -tip v4 0/6] kprobes: introduce NOKPROBE_SYMBOL() and fixes crash bugs
- From: "Frank Ch. Eigler" <fche at redhat dot com>
- To: Masami Hiramatsu <masami dot hiramatsu dot pt at hitachi dot com>
- Cc: Ingo Molnar <mingo at kernel dot org>, Ananth N Mavinakayanahalli <ananth at in dot ibm dot com>, Sandeepa Prabhu <sandeepa dot prabhu at linaro dot org>, x86 at kernel dot org, lkml <linux-kernel at vger dot kernel dot org>, "Steven Rostedt (Red Hat)" <rostedt at goodmis dot org>, systemtap at sourceware dot org, "David S. Miller" <davem at davemloft dot net>
- Date: Fri, 6 Dec 2013 20:32:49 -0500
- Subject: Re: [PATCH -tip v4 0/6] kprobes: introduce NOKPROBE_SYMBOL() and fixes crash bugs
- Authentication-results: sourceware.org; auth=none
- References: <20131204012841 dot 22118 dot 82992 dot stgit at kbuild-fedora dot novalocal> <20131204084551 dot GA31772 at gmail dot com> <529FBA71 dot 6070107 at hitachi dot com> <y0mvbz3pd3z dot fsf at fche dot csb> <52A16AD7 dot 6040500 at hitachi dot com> <20131206190753 dot GA3201 at redhat dot com> <52A25B71 dot 3090108 at hitachi dot com>
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.
> > 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?)
> > 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, ...).
> >> [...] 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.
- FChE