linux-kernel - Re: [PATCH v4 1/8] tracing: wprobe: Add watchpoint probe event based on hardware breakpoint

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Message-ID: <f8d432e0-e220-4857-8d4e-427332ce837a@infradead.org>
Date: Wed, 17 Sep 2025 10:13:36 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Steven Rostedt <rostedt@...dmis.org>,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>,
 x86@...nel.org, 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

On 9/17/25 7:38 AM, Masami Hiramatsu (Google) wrote:
>> +  #           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?

Let me try this again.

I understand the stack (call) trace/dump and your explanation, but
then your next comment says:

+You can see the code which writes to `jiffies` is `do_timer()`.

Does that need to be corrected?

thanks.
-- 
~Randy