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Re: [PATCH -tip v6 00/22] kprobes: introduce NOKPROBE_SYMBOL(), cleanup and fixes crash bugs
- From: Ingo Molnar <mingo at kernel dot org>
- To: Masami Hiramatsu <masami dot hiramatsu dot pt at hitachi dot com>
- Cc: "Frank Ch. Eigler" <fche at redhat dot com>, linux-arch at vger dot 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, 20 Dec 2013 11:46:15 +0100
- Subject: Re: [PATCH -tip v6 00/22] kprobes: introduce NOKPROBE_SYMBOL(), cleanup and fixes crash bugs
- Authentication-results: sourceware.org; auth=none
- References: <20131219090353 dot 14309 dot 15496 dot stgit at kbuild-fedora dot novalocal> <y0m38loefhx dot fsf at fche dot csb> <52B3C5E6 dot 2040802 at hitachi dot com> <20131220082056 dot GA15934 at gmail dot com> <52B40E79 dot 8040701 at hitachi dot com>
* Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com> wrote:
> (2013/12/20 17:20), Ingo Molnar wrote:
> >
> > * Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com> wrote:
> >
> >>> But a closer look indicates that the insertion of kprobes is
> >>> taking about three (!!) orders of magnitude longer than before, as
> >>> judged by the rate of increase of 'wc -l
> >>> /sys/kernel/debug/kprobes/list'.
> >>
> >> Right, because kprobes are not designed for thousands of probes.
> >
> > Then this needs to be fixed, because right now this bug is making it
> > near impossible to properly test kprobes robustness.
> >
> > For example a hash table (hashed by probe address) could be used in
> > addition to the list, to speed up basic operations.
>
> kprobe itself is already using hlist (6bits hash table).
> Maybe we'd better expand the table bits. However, the iteration
> of the list on debugfs is just doing seq_printf()s. I'm not exactly
> sure what Frank complaints about...
Well, Frank reported that the test he performed takes hours to finish,
and he mentioned a specific script line he used to produce that:
# stap -te "probe kprobe.function("*") {}"
I suspect an equivalent perf probe sequence would be something like:
# for FUNC in $(grep -iw t /proc/kallsyms | cut -d' ' -f3); do date; perf probe -a $FUNC; done
(totally untested.)
Can you reproduce that slowdown, using his method?
I can reproduce one weirdness, with just 13 probes added, 'perf probe
-l' [which should really be 'perf probe list'!] executes very slowly:
# perf stat --null --repeat 3 perf probe -l
Performance counter stats for 'perf probe -l' (3 runs):
0.763640098 seconds time elapsed ( +- 1.61% )
0.7 seconds is ridiculously long.
Also, here's another bugreport as well: while playing around with
'perf probe' I found that its usability is still very poor. For
example I mis-remembered the syntax and typed the obvious way to :
# perf probe add __schedule
Failed to find path of kernel module.
Failed to open debuginfo file.
Error: Failed to add events. (-2)
why the heck does a simple and obvious 'perf probe add' not work, why
is the strange syntax of 'perf probe -a' forced? Every other perf
subcommand uses clean command spaces - see for example 'perf bench'.
Also, the error message is totally misleading and uninformative to the
level of being passive-aggressive. An error message should directly
relate to the mistake performed and should give a good way out of the
situation. Who the heck cares that there was no debuginfo file to
open? Who cares that the 'path of kernel module' was not found? It has
no relation to the bug.
An informative error message would be:
# perf probe add __schedule
Error: Could not find symbol 'add'.
and that's it. No 'failed to add events' message - obviously the event
is not enabled if we cannot find the symbol name.
Thanks,
Ingo