tracing: Add kprobe-based event tracer documentation
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>hifive-unleashed-5.1
Masami Hiramatsu
Frederic Weisbecker
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Kprobe-based Event Tracer
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=========================
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Documentation is written by Masami Hiramatsu
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Overview
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--------
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This tracer is similar to the events tracer which is based on Tracepoint
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infrastructure. Instead of Tracepoint, this tracer is based on kprobes(kprobe
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and kretprobe). It probes anywhere where kprobes can probe(this means, all
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functions body except for __kprobes functions).
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Unlike the function tracer, this tracer can probe instructions inside of
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kernel functions. It allows you to check which instruction has been executed.
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Unlike the Tracepoint based events tracer, this tracer can add and remove
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probe points on the fly.
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Similar to the events tracer, this tracer doesn't need to be activated via
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current_tracer, instead of that, just set probe points via
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/sys/kernel/debug/tracing/kprobe_events. And you can set filters on each
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probe events via /sys/kernel/debug/tracing/events/kprobes/<EVENT>/filter.
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Synopsis of kprobe_events
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-------------------------
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p[:EVENT] SYMBOL[+offs|-offs]|MEMADDR [FETCHARGS] : Set a probe
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r[:EVENT] SYMBOL[+0] [FETCHARGS] : Set a return probe
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EVENT : Event name.
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SYMBOL[+offs|-offs] : Symbol+offset where the probe is inserted.
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MEMADDR : Address where the probe is inserted.
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FETCHARGS : Arguments.
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%REG : Fetch register REG
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sN : Fetch Nth entry of stack (N >= 0)
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sa : Fetch stack address.
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@ADDR : Fetch memory at ADDR (ADDR should be in kernel)
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@SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol)
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aN : Fetch function argument. (N >= 0)(*)
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rv : Fetch return value.(**)
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ra : Fetch return address.(**)
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+|-offs(FETCHARG) : fetch memory at FETCHARG +|- offs address.(***)
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(*) aN may not correct on asmlinkaged functions and at the middle of
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function body.
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(**) only for return probe.
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(***) this is useful for fetching a field of data structures.
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Per-Probe Event Filtering
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-------------------------
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Per-probe event filtering feature allows you to set different filter on each
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probe and gives you what arguments will be shown in trace buffer. If an event
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name is specified right after 'p:' or 'r:' in kprobe_events, the tracer adds
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an event under tracing/events/kprobes/<EVENT>, at the directory you can see
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'id', 'enabled', 'format' and 'filter'.
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enabled:
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You can enable/disable the probe by writing 1 or 0 on it.
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format:
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It shows the format of this probe event. It also shows aliases of arguments
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which you specified to kprobe_events.
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filter:
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You can write filtering rules of this event. And you can use both of aliase
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names and field names for describing filters.
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Usage examples
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--------------
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To add a probe as a new event, write a new definition to kprobe_events
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as below.
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echo p:myprobe do_sys_open a0 a1 a2 a3 > /sys/kernel/debug/tracing/kprobe_events
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This sets a kprobe on the top of do_sys_open() function with recording
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1st to 4th arguments as "myprobe" event.
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echo r:myretprobe do_sys_open rv ra >> /sys/kernel/debug/tracing/kprobe_events
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This sets a kretprobe on the return point of do_sys_open() function with
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recording return value and return address as "myretprobe" event.
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You can see the format of these events via
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/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format.
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cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format
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name: myprobe
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ID: 23
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format:
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field:unsigned short common_type; offset:0; size:2;
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field:unsigned char common_flags; offset:2; size:1;
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field:unsigned char common_preempt_count; offset:3; size:1;
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field:int common_pid; offset:4; size:4;
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field:int common_tgid; offset:8; size:4;
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field: unsigned long ip; offset:16;tsize:8;
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field: int nargs; offset:24;tsize:4;
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field: unsigned long arg0; offset:32;tsize:8;
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field: unsigned long arg1; offset:40;tsize:8;
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field: unsigned long arg2; offset:48;tsize:8;
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field: unsigned long arg3; offset:56;tsize:8;
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alias: a0; original: arg0;
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alias: a1; original: arg1;
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alias: a2; original: arg2;
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alias: a3; original: arg3;
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print fmt: "%lx: 0x%lx 0x%lx 0x%lx 0x%lx", ip, arg0, arg1, arg2, arg3
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You can see that the event has 4 arguments and alias expressions
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corresponding to it.
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echo > /sys/kernel/debug/tracing/kprobe_events
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This clears all probe points. and you can see the traced information via
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/sys/kernel/debug/tracing/trace.
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cat /sys/kernel/debug/tracing/trace
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# tracer: nop
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#
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# TASK-PID CPU# TIMESTAMP FUNCTION
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# | | | | |
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<...>-1447 [001] 1038282.286875: do_sys_open+0x0/0xd6: 0x3 0x7fffd1ec4440 0x8000 0x0
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<...>-1447 [001] 1038282.286878: sys_openat+0xc/0xe <- do_sys_open: 0xfffffffffffffffe 0xffffffff81367a3a
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<...>-1447 [001] 1038282.286885: do_sys_open+0x0/0xd6: 0xffffff9c 0x40413c 0x8000 0x1b6
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<...>-1447 [001] 1038282.286915: sys_open+0x1b/0x1d <- do_sys_open: 0x3 0xffffffff81367a3a
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<...>-1447 [001] 1038282.286969: do_sys_open+0x0/0xd6: 0xffffff9c 0x4041c6 0x98800 0x10
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<...>-1447 [001] 1038282.286976: sys_open+0x1b/0x1d <- do_sys_open: 0x3 0xffffffff81367a3a
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Each line shows when the kernel hits a probe, and <- SYMBOL means kernel
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returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
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returns from do_sys_open to sys_open+0x1b).
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