Message-ID: <014136d2-8599-4a1f-ab8e-c5be4f522e5a@infradead.org>
Date: Sun, 14 Sep 2025 17:14:37 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
 Steven Rostedt <rostedt@...dmis.org>, Peter Zijlstra <peterz@...radead.org>,
 Ingo Molnar <mingo@...nel.org>, x86@...nel.org
Cc: Jinchao Wang <wangjinchao600@...il.com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>, "H . Peter Anvin"
 <hpa@...or.com>, Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Ian Rogers <irogers@...gle.com>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v4 1/8] tracing: wprobe: Add watchpoint probe event based
 on hardware breakpoint

Hi,

On 9/14/25 7:09 AM, Masami Hiramatsu (Google) wrote:
> From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> 
> Add a new probe event for the hardware breakpoint called wprobe-event.
> This wprobe allows user to trace (watch) the memory access at the
> specified memory address.
> The new syntax is;
> 
>  w[:[GROUP/]EVENT] [r|w|rw]@[ADDR|SYM][:SIZE] [FETCH_ARGs]
> 
> User also can use $addr to fetch the accessed address. But no other
> variables are supported. To record updated value, use '+0($addr)'.
> 
> For example, tracing updates of the jiffies;
> 
>  /sys/kernel/tracing # echo 'w:my_jiffies w@...fies' >> dynamic_events
>  /sys/kernel/tracing # cat dynamic_events
>  w:wprobes/my_jiffies w@...fies:4
>  /sys/kernel/tracing # echo 1 > events/wprobes/my_jiffies/enable
>  /sys/kernel/tracing # head -n 20 trace | tail -n 5
>  #           TASK-PID     CPU#  |||||  TIMESTAMP  FUNCTION
>  #              | |         |   |||||     |         |
>           <idle>-0       [000] d.Z1.   206.547317: my_jiffies: (tick_do_update_jiffies64+0xbe/0x130)
>           <idle>-0       [000] d.Z1.   206.548341: my_jiffies: (tick_do_update_jiffies64+0xbe/0x130)
>           <idle>-0       [000] d.Z1.   206.549346: my_jiffies: (tick_do_update_jiffies64+0xbe/0x130)
> 
> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> ---
>  Changes in v3:
>    - Enclose the register-API dependent code in trace_probe.c with
>      CONFIG_HAVE_FUNCTION_ARG_ACCESS_API.
> ---
>  Documentation/trace/index.rst       |    1 
>  Documentation/trace/wprobetrace.rst |   69 ++++
>  include/linux/trace_events.h        |    2 
>  kernel/trace/Kconfig                |   14 +
>  kernel/trace/Makefile               |    1 
>  kernel/trace/trace.c                |    9 
>  kernel/trace/trace.h                |    5 
>  kernel/trace/trace_probe.c          |   22 +
>  kernel/trace/trace_probe.h          |    8 
>  kernel/trace/trace_wprobe.c         |  685 +++++++++++++++++++++++++++++++++++
>  10 files changed, 813 insertions(+), 3 deletions(-)
>  create mode 100644 Documentation/trace/wprobetrace.rst
>  create mode 100644 kernel/trace/trace_wprobe.c
> 


> diff --git a/Documentation/trace/wprobetrace.rst b/Documentation/trace/wprobetrace.rst
> new file mode 100644
> index 000000000000..9774f57e2947
> --- /dev/null
> +++ b/Documentation/trace/wprobetrace.rst
> @@ -0,0 +1,69 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +=======================================
> +Watchpoint probe (wprobe) Event Tracing
> +=======================================
> +
> +.. Author: Masami Hiramatsu <mhiramat@...nel.org>
> +
> +Overview
> +--------
> +
> +Wprobe event is a dynamic event based on the hardware breakpoint, which is
> +similar to other probe events, but it is for watching data access. It allows
> +you to trace which code accesses a specified data.
> +
> +As same as other dynamic events, wprobe events are defined via
> +`dynamic_events` interface file on tracefs.
> +
> +Synopsis of wprobe-events
> +-------------------------
> +::
> +
> +  w:[GRP/][EVENT] SPEC [FETCHARGS]                       : Probe on data access
> +
> + GRP            : Group name for wprobe. If omitted, use "wprobes" for it.
> + EVENT          : Event name for wprobe. If omitted, an event name is
> +                  generated based on the address or symbol.
> + SPEC           : Breakpoint specification.
> +                  [r|w|rw]@<ADDRESS|SYMBOL[+|-OFFS]>[:LENGTH]
> +
> +   r|w|rw       : Access type, r for read, w for write, and rw for both.
> +                  Use rw if omitted.

		     Default is rw if omitted.

> +   ADDRESS      : Address to trace (hexadecimal).
> +   SYMBOL       : Symbol name to trace.
> +   LENGTH       : Length of the data to trace in bytes. (1, 2, 4, or 8)
> +
> + FETCHARGS      : Arguments. Each probe can have up to 128 args.
> +  $addr         : Fetch the accessing address.
> +  @ADDR         : Fetch memory at ADDR (ADDR should be in kernel)
> +  @SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol)
> +  +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*1)(\*2)
> +  \IMM          : Store an immediate value to the argument.
> +  NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
> +  FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
> +                  (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
> +                  (x8/x16/x32/x64), "char", "string", "ustring", "symbol", "symstr"
> +                  and bitfield are supported.
> +
> +  (\*1) this is useful for fetching a field of data structures.
> +  (\*2) "u" means user-space dereference.
> +
> +For the details of TYPE, see :ref:`kprobetrace documentation <kprobetrace_types>`.
> +
> +Usage examples
> +--------------
> +Here is an example to add a wprobe event on a variable `jiffies`.
> +::
> +
> +  # echo 'w:my_jiffies w@...fies' >> dynamic_events
> +  # cat dynamic_events
> +  w:wprobes/my_jiffies w@...fies
> +  # echo 1 > events/wprobes/enable
> +  # cat trace | head
> +  #           TASK-PID     CPU#  |||||  TIMESTAMP  FUNCTION
> +  #              | |         |   |||||     |         |
> +           <idle>-0       [000] d.Z1.  717.026259: my_jiffies: (tick_do_update_jiffies64+0xbe/0x130)
> +           <idle>-0       [000] d.Z1.  717.026373: my_jiffies: (tick_do_update_jiffies64+0xbe/0x130)
> +
> +You can see the code which writes to `jiffies` is `do_timer()`.

I'm having trouble getting from tick_do_update_jiffies64+0xbe/0x130,
which I expect is
	jiffies_64 += ticks;
in that function, over to do_timer(), which also updates jiffies_64,
but is not called by tick_do_update_jiffies64(). AFAICT, there are
no calls to do_timer() in the file (kernel/time/tick-sched.c).

Can you explain, please?



> diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> index d2c79da81e4f..dd8919386425 100644
> --- a/kernel/trace/Kconfig
> +++ b/kernel/trace/Kconfig
> @@ -807,6 +807,20 @@ config EPROBE_EVENTS
>  	  convert the type of an event field. For example, turn an
>  	  address into a string.
>  
> +config WPROBE_EVENTS
> +	bool "Enable wprobe-based dynamic events"
> +	depends on TRACING
> +	depends on HAVE_HW_BREAKPOINT
> +	select PROBE_EVENTS
> +	select DYNAMIC_EVENTS
> +	default y

Wny default y?

> +	help
> +	  This allows the user to add watchpoint tracing events based on
> +	  hardware breakpoints on the fly via the ftrace interface.
> +
> +	  Those events can be inserted wherever hardware breakpoints can be
> +	  set, and record various register and memory values.
> +
>  config BPF_EVENTS
>  	depends on BPF_SYSCALL
>  	depends on (KPROBE_EVENTS || UPROBE_EVENTS) && PERF_EVENTS


thanks.
-- 
~Randy

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