linux-kernel - Re: [PATCH 6/8] tracing: Have persistent trace instances save module addresses

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Message-Id: <20250207015307.ce9273f4cd5eee9335c335d3@kernel.org>
Date: Fri, 7 Feb 2025 01:53:07 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Mark
 Rutland <mark.rutland@....com>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 6/8] tracing: Have persistent trace instances save
 module addresses

On Thu, 6 Feb 2025 10:29:49 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Thu, 6 Feb 2025 17:26:42 +0900
> Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:
> 
> > This maybe too frequently scan the module because the update_last_data() is
> > called when;
> > - change tracer (this maybe OK)
> > - update "set_event"
> > - write 1 to "enable" under events
> > - change pid filter
> > - etc.
> > 
> > Once it is scanned, it should not scan again, but update by module
> > callback, because usually events are enabled individually (thus
> > write 1 to "event" can happen multiple time online).
> > 
> > I think we can move this after TRACE_ARRAY_FL_LAST_BOOT check,
> > if TRACE_ARRAY_FL_LAST_BOOT flag is set, that flag should be cleared
> > with updating the tscratch, and the flag is not set, we can skip updating
> > the scratch.
> 
> No, because we could enable it, disable it, and then the user could be
> adding and removing modules all day long, and we only add and do not remove
> (I do have a patch that does remove them).

I think TRACE_ARRAY_FL_LAST_BOOT is something like one-way flag, it is set
if the buffer is persistent, but cleared when it is reused (start writing)
and never be back. Isn't it?

> 
> But we can do the same check as patch 8 does, and only do this if the
> tracer wasn't already active.

Yeah, so I think this change needs patch 8 change.

Thank you,

> 
> -- Steve


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>