linux-kernel - Re: [GIT PULL] tracing: Updates for v6.13

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Message-ID: <20241122171323.1dd0efc9@gandalf.local.home>
Date: Fri, 22 Nov 2024 17:13:23 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Andrii Nakryiko <andrii@...nel.org>, Colin Ian King
 <colin.i.king@...il.com>, Jeff Xie <jeff.xie@...ux.dev>, Jinjie Ruan
 <ruanjinjie@...wei.com>, Josh Poimboeuf <jpoimboe@...nel.org>, Justin Stitt
 <justinstitt@...gle.com>, Levi Yun <yeoreum.yun@....com>, Li Chen
 <chenl311@...natelecom.cn>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Ryan Roberts <ryan.roberts@....com>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Tatsuya S
 <tatsuya.s2862@...il.com>, Uros Bizjak <ubizjak@...il.com>, Zheng Yejian
 <zhengyejian@...weicloud.com>, guoweikang <guoweikang.kernel@...il.com>
Subject: Re: [GIT PULL] tracing: Updates for v6.13

On Fri, 22 Nov 2024 13:54:49 -0800
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> And yes, it's a static conditional, but it's *STUPID*. And it makes
> the code just harder to follow, for no good reason. The difference is
> literally this magic
> 
>                         __DO_TRACE(name,                                \
>                                 TP_ARGS(args),                          \
>                                 TP_CONDITION(cond), 0);                 \
> 
> vs
> 
>                         __DO_TRACE(name,                                \
>                                 TP_ARGS(args),                          \
>                                 TP_CONDITION(cond), 1);                 \

Hmm, if we make a __DO_TRACE_SYSCALL(), I don't think it needs to even have
that condition parameter. The TP_CONDITION() is to allow tracepoints to
check a condition within the static_branch() portion of the tracepoint so
the condition does not get called when tracepoints are disabled (and just a
nop).  By default we have:

#define DECLARE_TRACE_SYSCALL(name, proto, args)			\
	__DECLARE_TRACE_SYSCALL(name, PARAMS(proto), PARAMS(args),	\
				cpu_online(raw_smp_processor_id()),	\
				PARAMS(void *__data, proto))


Where the tracepoint has to at least be called when the current CPU is
online. But is there every a case where a CPU is not online when a system
call is entered or exited?

And there isn't any use cases of calling the system call tracepoint with a
condition. Hence, I think the __DO_TRACE_SYSCALL() doesn't even need to
include the condition:

#define __DO_TRACE_SYSCALL(name, args)					\
	do {								\
		int __maybe_unused __idx = 0;				\

Oh, also the __idx isn't used since I removed the SRCU, so we can remove
that too. Maybe make that a separate patch?

		rcu_read_lock_trace();					\
									\
		__DO_TRACE_CALL(name, TP_ARGS(args));			\
									\
		rcu_read_unlock_trace();				\
	} while (0)

-- Steve