Re: [RFC PATCH tip/master 0/3] kprobes: tracing: kretprobe_instance dynamic allocation

[Frank Ch. Eigler]

mhiramat wrote:

> Here is a correction of patches to introduce kretprobe_instance
> dynamic allocation for avoiding kretprobe silently miss-hits.
> [...]

Thanks, this looks automatically useful also to systemtap users.

- FChE
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[RFC PATCH tip/master 0/3] kprobes: tracing: kretprobe_instance dynamic allocation

[Masami Hiramatsu]

Here is a correction of patches to introduce kretprobe_instance
dynamic allocation for avoiding kretprobe silently miss-hits.

Original issue was reported by Lukasz on linux-trace ml.
 https://www.spinics.net/lists/linux-trace/msg00448.html

Also Alban is working on kprobe-tracer side because of
iovisor's issue.
 https://www.spinics.net/lists/netdev/msg427149.html
(Note that this series is independently applicable, no conflict)

This series is a kind of complementary patches for
Alban's patch. So I think both of them are needed.

 [1/3]: Add kretprobe's miss-hit counter to miss-hit
column on kprobe_profile. This helps user to
see what happened.
 [2/3]: Introduce kretprobe_instance dynamic allocation.
This will help user not to miss the ret probes
even it has low number of maxactive.
 [3/3]: Set maximum limitation for pre-allocated maxactive.
This can avoid miss configuration of struct kretprobe.

The downside of this patch is, dynamic allocation will
involve memory allocation, which sometimes traced by
kprobes. In that case those nested kprobes are missed.
To avoid this kind of miss-hit, user may need to make
maxactive enough large when registering kretprobes.

However, in other case, this can reduce the possibility
of miss-hit of kretprobes. Since the maxactive increased
automatically, user will not need to retry tracing with
larger maxactive.

Alban, you can reuse KRETPROBE_MAXACTIVE_ALLOC for checking
upper limiation in trace_kprobe.c too.

Thank you,

---

Masami Hiramatsu (3):
  trace: kprobes: Show sum of probe/retprobe nmissed count
  kprobes: Allocate kretprobe instance if its free list is empty
  kprobes: Limit kretprobe maximum instances


 Documentation/kprobes.txt   |   25 +++-
 include/linux/kprobes.h |2 ++
 kernel/kprobes.c|   44 +++
 kernel/trace/trace_kprobe.c |2 +-
 4 files changed, 50 insertions(+), 23 deletions(-)

--
Masami Hiramatsu (Linaro) 
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