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    Subject[PATCH 09/18] tracing: Add kprobe-based event tracer documentation
    Date
    From: Masami Hiramatsu <mhiramat@redhat.com>

    Add the documentation to use the kprobe based event tracer.

    [fweisbec@gmail.com: Split tracer and its Documentation in two patchs]

    Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
    Acked-by: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
    Cc: Avi Kivity <avi@redhat.com>
    Cc: Andi Kleen <ak@linux.intel.com>
    Cc: Christoph Hellwig <hch@infradead.org>
    Cc: Frank Ch. Eigler <fche@redhat.com>
    Cc: H. Peter Anvin <hpa@zytor.com>
    Cc: Ingo Molnar <mingo@elte.hu>
    Cc: Jason Baron <jbaron@redhat.com>
    Cc: Jim Keniston <jkenisto@us.ibm.com>
    Cc: K.Prasad <prasad@linux.vnet.ibm.com>
    Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
    Cc: Li Zefan <lizf@cn.fujitsu.com>
    Cc: Przemysław Pawełczyk <przemyslaw@pawelczyk.it>
    Cc: Roland McGrath <roland@redhat.com>
    Cc: Sam Ravnborg <sam@ravnborg.org>
    Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
    Cc: Steven Rostedt <rostedt@goodmis.org>
    Cc: Tom Zanussi <tzanussi@gmail.com>
    Cc: Vegard Nossum <vegard.nossum@gmail.com>
    LKML-Reference: <20090813203510.31965.29123.stgit@localhost.localdomain>
    Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    ---
    Documentation/trace/kprobetrace.txt | 139 +++++++++++++++++++++++++++++++++++
    1 files changed, 139 insertions(+), 0 deletions(-)
    create mode 100644 Documentation/trace/kprobetrace.txt

    diff --git a/Documentation/trace/kprobetrace.txt b/Documentation/trace/kprobetrace.txt
    new file mode 100644
    index 0000000..efff6eb
    --- /dev/null
    +++ b/Documentation/trace/kprobetrace.txt
    @@ -0,0 +1,139 @@
    + Kprobe-based Event Tracer
    + =========================
    +
    + Documentation is written by Masami Hiramatsu
    +
    +
    +Overview
    +--------
    +This tracer is similar to the events tracer which is based on Tracepoint
    +infrastructure. Instead of Tracepoint, this tracer is based on kprobes(kprobe
    +and kretprobe). It probes anywhere where kprobes can probe(this means, all
    +functions body except for __kprobes functions).
    +
    +Unlike the function tracer, this tracer can probe instructions inside of
    +kernel functions. It allows you to check which instruction has been executed.
    +
    +Unlike the Tracepoint based events tracer, this tracer can add and remove
    +probe points on the fly.
    +
    +Similar to the events tracer, this tracer doesn't need to be activated via
    +current_tracer, instead of that, just set probe points via
    +/sys/kernel/debug/tracing/kprobe_events. And you can set filters on each
    +probe events via /sys/kernel/debug/tracing/events/kprobes/<EVENT>/filter.
    +
    +
    +Synopsis of kprobe_events
    +-------------------------
    + p[:EVENT] SYMBOL[+offs|-offs]|MEMADDR [FETCHARGS] : Set a probe
    + r[:EVENT] SYMBOL[+0] [FETCHARGS] : Set a return probe
    +
    + EVENT : Event name.
    + SYMBOL[+offs|-offs] : Symbol+offset where the probe is inserted.
    + MEMADDR : Address where the probe is inserted.
    +
    + FETCHARGS : Arguments.
    + %REG : Fetch register REG
    + sN : Fetch Nth entry of stack (N >= 0)
    + sa : Fetch stack address.
    + @ADDR : Fetch memory at ADDR (ADDR should be in kernel)
    + @SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol)
    + aN : Fetch function argument. (N >= 0)(*)
    + rv : Fetch return value.(**)
    + ra : Fetch return address.(**)
    + +|-offs(FETCHARG) : fetch memory at FETCHARG +|- offs address.(***)
    +
    + (*) aN may not correct on asmlinkaged functions and at the middle of
    + function body.
    + (**) only for return probe.
    + (***) this is useful for fetching a field of data structures.
    +
    +
    +Per-Probe Event Filtering
    +-------------------------
    + Per-probe event filtering feature allows you to set different filter on each
    +probe and gives you what arguments will be shown in trace buffer. If an event
    +name is specified right after 'p:' or 'r:' in kprobe_events, the tracer adds
    +an event under tracing/events/kprobes/<EVENT>, at the directory you can see
    +'id', 'enabled', 'format' and 'filter'.
    +
    +enabled:
    + You can enable/disable the probe by writing 1 or 0 on it.
    +
    +format:
    + It shows the format of this probe event. It also shows aliases of arguments
    + which you specified to kprobe_events.
    +
    +filter:
    + You can write filtering rules of this event. And you can use both of aliase
    + names and field names for describing filters.
    +
    +
    +Usage examples
    +--------------
    +To add a probe as a new event, write a new definition to kprobe_events
    +as below.
    +
    + echo p:myprobe do_sys_open a0 a1 a2 a3 > /sys/kernel/debug/tracing/kprobe_events
    +
    + This sets a kprobe on the top of do_sys_open() function with recording
    +1st to 4th arguments as "myprobe" event.
    +
    + echo r:myretprobe do_sys_open rv ra >> /sys/kernel/debug/tracing/kprobe_events
    +
    + This sets a kretprobe on the return point of do_sys_open() function with
    +recording return value and return address as "myretprobe" event.
    + You can see the format of these events via
    +/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format.
    +
    + cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format
    +name: myprobe
    +ID: 23
    +format:
    + field:unsigned short common_type; offset:0; size:2;
    + field:unsigned char common_flags; offset:2; size:1;
    + field:unsigned char common_preempt_count; offset:3; size:1;
    + field:int common_pid; offset:4; size:4;
    + field:int common_tgid; offset:8; size:4;
    +
    + field: unsigned long ip; offset:16;tsize:8;
    + field: int nargs; offset:24;tsize:4;
    + field: unsigned long arg0; offset:32;tsize:8;
    + field: unsigned long arg1; offset:40;tsize:8;
    + field: unsigned long arg2; offset:48;tsize:8;
    + field: unsigned long arg3; offset:56;tsize:8;
    +
    + alias: a0; original: arg0;
    + alias: a1; original: arg1;
    + alias: a2; original: arg2;
    + alias: a3; original: arg3;
    +
    +print fmt: "%lx: 0x%lx 0x%lx 0x%lx 0x%lx", ip, arg0, arg1, arg2, arg3
    +
    +
    + You can see that the event has 4 arguments and alias expressions
    +corresponding to it.
    +
    + echo > /sys/kernel/debug/tracing/kprobe_events
    +
    + This clears all probe points. and you can see the traced information via
    +/sys/kernel/debug/tracing/trace.
    +
    + cat /sys/kernel/debug/tracing/trace
    +# tracer: nop
    +#
    +# TASK-PID CPU# TIMESTAMP FUNCTION
    +# | | | | |
    + <...>-1447 [001] 1038282.286875: do_sys_open+0x0/0xd6: 0x3 0x7fffd1ec4440 0x8000 0x0
    + <...>-1447 [001] 1038282.286878: sys_openat+0xc/0xe <- do_sys_open: 0xfffffffffffffffe 0xffffffff81367a3a
    + <...>-1447 [001] 1038282.286885: do_sys_open+0x0/0xd6: 0xffffff9c 0x40413c 0x8000 0x1b6
    + <...>-1447 [001] 1038282.286915: sys_open+0x1b/0x1d <- do_sys_open: 0x3 0xffffffff81367a3a
    + <...>-1447 [001] 1038282.286969: do_sys_open+0x0/0xd6: 0xffffff9c 0x4041c6 0x98800 0x10
    + <...>-1447 [001] 1038282.286976: sys_open+0x1b/0x1d <- do_sys_open: 0x3 0xffffffff81367a3a
    +
    +
    + Each line shows when the kernel hits a probe, and <- SYMBOL means kernel
    +returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
    +returns from do_sys_open to sys_open+0x1b).
    +
    +
    --
    1.6.2.3
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