Re: [PATCH v2] tracing/kprobes: expose maxactive for kretprobe in kprobe_events

[Masami Hiramatsu <mhiramat@kernel.org>]

On Fri, 31 Mar 2017 10:08:39 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Fri, 31 Mar 2017 15:20:24 +0200
> Alban Crequy <alban.crequy@gmail.com> wrote:
> 
> > When a kretprobe is installed on a kernel function, there is a maximum
> > limit of how many calls in parallel it can catch (aka "maxactive"). A
> > kernel module could call register_kretprobe() and initialize maxactive
> > (see example in samples/kprobes/kretprobe_example.c).
> > 
> > But that is not exposed to userspace and it is currently not possible to
> > choose maxactive when writing to /sys/kernel/debug/tracing/kprobe_events
> > 
> > The default maxactive can be as low as 1 on single-core with a
> > non-preemptive kernel. This is too low and we need to increase it not
> > only for recursive functions, but for functions that sleep or resched.
> > 
> > This patch updates the format of the command that can be written to
> > kprobe_events so that maxactive can be optionally specified.
> > 
> > I need this for a bpf program attached to the kretprobe of
> > inet_csk_accept, which can sleep for a long time.
> > 
> > This patch includes a basic selftest:
> > 
> > > # ./ftracetest -v  test.d/kprobe/
> > > === Ftrace unit tests ===
> > > [1] Kprobe dynamic event - adding and removing	[PASS]
> > > [2] Kprobe dynamic event - busy event check	[PASS]
> > > [3] Kprobe dynamic event with arguments	[PASS]
> > > [4] Kprobes event arguments with types	[PASS]
> > > [5] Kprobe dynamic event with function tracer	[PASS]
> > > [6] Kretprobe dynamic event with arguments	[PASS]
> > > [7] Kretprobe dynamic event with maxactive	[PASS]
> > >
> > > # of passed:  7
> > > # of failed:  0
> > > # of unresolved:  0
> > > # of untested:  0
> > > # of unsupported:  0
> > > # of xfailed:  0
> > > # of undefined(test bug):  0  
> > 
> > BugLink: https://github.com/iovisor/bcc/issues/1072
> > Signed-off-by: Alban Crequy <alban@kinvolk.io>
> > 
> > ---
> > 
> > Changes since v1:
> > - Remove "(*)" from documentation. (Review from Masami Hiramatsu)
> > - Fix support for "r100" without the event name (Review from Masami Hiramatsu)
> > - Get rid of magic numbers within the code.  (Review from Steven Rostedt)
> >   Note that I didn't use KRETPROBE_MAXACTIVE_ALLOC since that patch is not
> >   merged.
> > - Return -E2BIG when maxactive is too big.
> > - Add basic selftest
> > ---
> >  Documentation/trace/kprobetrace.txt                |  4 ++-
> >  kernel/trace/trace_kprobe.c                        | 39 ++++++++++++++++++----
> >  .../ftrace/test.d/kprobe/kretprobe_maxactive.tc    | 39 ++++++++++++++++++++++
> >  3 files changed, 75 insertions(+), 7 deletions(-)
> >  create mode 100644 tools/testing/selftests/ftrace/test.d/kprobe/kretprobe_maxactive.tc
> > 
> > diff --git a/Documentation/trace/kprobetrace.txt b/Documentation/trace/kprobetrace.txt
> > index 41ef9d8..7051a20 100644
> > --- a/Documentation/trace/kprobetrace.txt
> > +++ b/Documentation/trace/kprobetrace.txt
> > @@ -23,7 +23,7 @@ current_tracer. Instead of that, add probe points via
> >  Synopsis of kprobe_events
> >  -------------------------
> >    p[:[GRP/]EVENT] [MOD:]SYM[+offs]|MEMADDR [FETCHARGS]	: Set a probe
> > -  r[:[GRP/]EVENT] [MOD:]SYM[+0] [FETCHARGS]		: Set a return probe
> > +  r[MAXACTIVE][:[GRP/]EVENT] [MOD:]SYM[+0] [FETCHARGS]	: Set a return probe
> >    -:[GRP/]EVENT						: Clear a probe
> >  
> >   GRP		: Group name. If omitted, use "kprobes" for it.
> > @@ -32,6 +32,8 @@ Synopsis of kprobe_events
> >   MOD		: Module name which has given SYM.
> >   SYM[+offs]	: Symbol+offset where the probe is inserted.
> >   MEMADDR	: Address where the probe is inserted.
> > + MAXACTIVE	: Maximum number of instances of the specified function that
> > +		  can be probed simultaneously, or 0 for the default.
> 
> BTW, to me, 0 means none (no instances can probe). This should have a
> better description of what "0" actually means.

default value is defined in Documentation/kprobes.txt sction 1.3.1, so
you'll just need to refer that.

Thank you,

> 
> -- Steve
> 
> 
> >  
> >   FETCHARGS	: Arguments. Each probe can have up to 128 args.
> >    %REG		: Fetch register REG


-- 
Masami Hiramatsu <mhiramat@kernel.org>