Linux-mm Archive on lore.kernel.org
 help / color / Atom feed
From: Steven Rostedt <rostedt@goodmis.org>
To: linux-kernel@vger.kernel.org
Cc: Yafang Shao <laoar.shao@gmail.com>,
	Axel Rasmussen <axelrasmussen@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Michel Lespinasse <walken@google.com>,
	Daniel Jordan <daniel.m.jordan@oracle.com>,
	Davidlohr Bueso <dbueso@suse.de>, Linux MM <linux-mm@kvack.org>,
	Ingo Molnar <mingo@kernel.org>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>
Subject: [PATCH 1/3 v2] tracepoints: Add helper to test if tracepoint is enabled in a header
Date: Fri, 25 Sep 2020 17:12:07 -0400
Message-ID: <20200925211819.767526657@goodmis.org> (raw)
In-Reply-To: <20200925211206.423598568@goodmis.org>

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

As tracepoints are discouraged from being added in a header because it can
cause side effects if other tracepoints are in headers, as well as bloat the
kernel as the trace_<tracepoint>() function is not a small inline, the common
workaround is to add a function call that calls a wrapper function in a
C file that then calls the tracepoint. But as function calls add overhead,
this function should only be called when the tracepoint in question is
enabled. To get around this overhead, a static_branch can be used to only
have the tracepoint wrapper get called when the tracepoint is enabled.

Add a tracepoint_enabled(tp) macro that gets passed the name of the
tracepoint, and this becomes a static_branch that is enabled when the
tracepoint is enabled and is a nop when the tracepoint is disabled.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 Documentation/trace/tracepoints.rst | 27 +++++++++++++++++++++++
 include/linux/tracepoint-defs.h     | 34 +++++++++++++++++++++++++++++
 2 files changed, 61 insertions(+)

diff --git a/Documentation/trace/tracepoints.rst b/Documentation/trace/tracepoints.rst
index 6e3ce3bf3593..68579ebd1e4c 100644
--- a/Documentation/trace/tracepoints.rst
+++ b/Documentation/trace/tracepoints.rst
@@ -146,3 +146,30 @@ with jump labels and avoid conditional branches.
       define tracepoints. Check http://lwn.net/Articles/379903,
       http://lwn.net/Articles/381064 and http://lwn.net/Articles/383362
       for a series of articles with more details.
+
+If you require calling a tracepoint from a header file, it is not
+recommended to call one directly or to use the trace_<tracepoint>_enabled()
+function call, as tracepoints in header files can have side effects if a
+header is included from a file that has CREATE_TRACE_POINTS set, as
+well as the trace_<tracepoint>() is not that small of an inline
+and can bloat the kernel if used by other inlined functions. Instead,
+include tracepoint-defs.h and use tracepoint_enabled().
+
+In a C file::
+
+	void do_trace_foo_bar_wrapper(args)
+	{
+		trace_foo_bar(args);
+	}
+
+In the header file::
+
+	DECLEARE_TRACEPOINT(foo_bar);
+
+	static inline void some_inline_function()
+	{
+		[..]
+		if (tracepoint_enabled(foo_bar))
+			do_trace_foo_bar_wrapper(args);
+		[..]
+	}
diff --git a/include/linux/tracepoint-defs.h b/include/linux/tracepoint-defs.h
index b29950a19205..60625973faaf 100644
--- a/include/linux/tracepoint-defs.h
+++ b/include/linux/tracepoint-defs.h
@@ -48,4 +48,38 @@ struct bpf_raw_event_map {
 	u32			writable_size;
 } __aligned(32);
 
+/*
+ * If a tracepoint needs to be called from a header file, it is not
+ * recommended to call it directly, as tracepoints in header files
+ * may cause side-effects and bloat the kernel. Instead, use
+ * tracepoint_enabled() to test if the tracepoint is enabled, then if
+ * it is, call a wrapper function defined in a C file that will then
+ * call the tracepoint.
+ *
+ * For "trace_foo_bar()", you would need to create a wrapper function
+ * in a C file to call trace_foo_bar():
+ *   void do_trace_foo_bar(args) { trace_foo_bar(args); }
+ * Then in the header file, declare the tracepoint:
+ *   DECLARE_TRACEPOINT(foo_bar);
+ * And call your wrapper:
+ *   static inline void some_inlined_function() {
+ *            [..]
+ *            if (tracepoint_enabled(foo_bar))
+ *                    do_trace_foo_bar(args);
+ *            [..]
+ *   }
+ *
+ * Note: tracepoint_enabled(foo_bar) is equivalent to trace_foo_bar_enabled()
+ *   but is safe to have in headers, where trace_foo_bar_enabled() is not.
+ */
+#define DECLARE_TRACEPOINT(tp) \
+	extern struct tracepoint __tracepoint_##tp
+
+#ifdef CONFIG_TRACEPOINTS
+# define tracepoint_enabled(tp) \
+	static_key_false(&(__tracepoint_##tp).key)
+#else
+# define tracepoint_enabled(tracepoint) false
+#endif
+
 #endif
-- 
2.28.0




  reply index

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-25 21:12 [PATCH 0/3 v2] tracing/mm: Add tracepoint_enabled() helper function for headers Steven Rostedt
2020-09-25 21:12 ` Steven Rostedt [this message]
2020-09-25 21:36   ` [PATCH 1/3 v2] tracepoints: Add helper to test if tracepoint is enabled in a header Axel Rasmussen
2020-09-25 21:59     ` Steven Rostedt
2020-10-20 11:59   ` Vlastimil Babka
2020-10-20 20:43     ` Steven Rostedt
2020-09-25 21:12 ` [PATCH 2/3 v2] mm/page_ref: Convert the open coded tracepoint enabled to the new helper Steven Rostedt
2020-10-20 12:10   ` Vlastimil Babka
2020-09-25 21:12 ` [PATCH 3/3 v2] x86: Use tracepoint_enabled() for msr tracepoints instead of open coding it Steven Rostedt
2020-09-26  6:55   ` kernel test robot
2020-09-25 21:26 ` [PATCH 0/3 v2] tracing/mm: Add tracepoint_enabled() helper function for headers Steven Rostedt
2020-09-25 22:23   ` Mathieu Desnoyers

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200925211819.767526657@goodmis.org \
    --to=rostedt@goodmis.org \
    --cc=akpm@linux-foundation.org \
    --cc=axelrasmussen@google.com \
    --cc=daniel.m.jordan@oracle.com \
    --cc=dbueso@suse.de \
    --cc=iamjoonsoo.kim@lge.com \
    --cc=laoar.shao@gmail.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=mingo@kernel.org \
    --cc=vbabka@suse.cz \
    --cc=walken@google.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-mm Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-mm/0 linux-mm/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-mm linux-mm/ https://lore.kernel.org/linux-mm \
		linux-mm@kvack.org
	public-inbox-index linux-mm

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kvack.linux-mm


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git