linux-kernel - Re: [FYI][PATCH] tracing/treewide: Remove second parameter of __assign

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Date: Fri, 23 Feb 2024 15:03:39 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Jeff Johnson <quic_jjohnson@...cinc.com>, LKML
 <linux-kernel@...r.kernel.org>, Linux Trace Kernel
 <linux-trace-kernel@...r.kernel.org>, Masami Hiramatsu
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 linux-s390@...r.kernel.org, tipc-discussion@...ts.sourceforge.net, Julia
 Lawall <Julia.Lawall@...ia.fr>
Subject: Re: [FYI][PATCH] tracing/treewide: Remove second parameter of
 __assign_str()

On Fri, 23 Feb 2024 14:50:49 -0500
Kent Overstreet <kent.overstreet@...ux.dev> wrote:

> Tangentially related though, what would make me really happy is if we
> could create the string with in the TP__fast_assign() section. I have to
> have a bunch of annoying wrappers right now because the string length
> has to be known when we invoke the tracepoint.

You can use __string_len() to determine the string length in the tracepoint
(which is executed in the TP_fast_assign() section).

My clean up patches will make __assign_str_len() obsolete too (I'm working
on them now), and you can just use __assign_str().

I noticed that I don't have a string_len example in the sample code and I'm
actually writing it now.

// cutting out everything else:

TRACE_EVENT(foo_bar,

	TP_PROTO(const char *foo, int bar),

	TP_ARGS(foo, bar),

	TP_STRUCT__entry(
		__string_len(	lstr,	foo,	bar < strlen(foo) ? bar : strlen(foo) )
	),

	TP_fast_assign(
		__assign_str(lstr, foo);

// Note, the above is with my updates, without them, you need to duplicate the logic

//		__assign_str_len(lstr, foo, bar < strlen(foo) ? bar : strlen(foo));
	),

	TP_printk("%s", __get_str(lstr))
);


The above will allocate "bar < strlen(foo) ? bar : strlen(foo)" size on the
ring buffer. As the size is already stored, my clean up code uses that
instead of requiring duplicating the logic again.

-- Steve