linux-kernel - Re: Interface for enabling context tracking

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Message-ID: <Z_mO6_m0bV-Q8NEa@pavilion.home>
Date: Fri, 11 Apr 2025 23:51:39 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Junxuan Liao <ljx@...wisc.edu>
Cc: Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: Interface for enabling context tracking

Le Fri, Apr 11, 2025 at 12:41:37PM -0500, Junxuan Liao a écrit :
> 
> > > Are you interested in working on joining the noble quest of getting the
> > > rest of the nohz_full support in place?  (Full disclosure: This stuff
> > > is non-trivial.)
> > 
> > I believe the request is more of just tracing entry and exit from the
> > kernel, which just needs a simple trace event at the border crossings.
> 
> Yeah I'm more interested in just tracing this for now.
> 
> > It's been on my todo list to add one for page
> > fault exit (as I do care for how long they last.
> 
> I've added a tracepoint similar to page_fault_user for that but I'm not
> sure if it's the best way to do it. Should I send a patch for review?

If you do so, it may be a good idea to remove page_fault_user and
page_fault_kernel and introduce page_fault_user_enter/page_fault_user_exit
and page_fault_kernel_enter/page_fault_kernel_exit.

But the following is also possible (and then trace/events/context_tracking.h
should be renamed into trace/events/entry.h):

diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
index fc61d0205c97..83b1764078f7 100644
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -15,6 +15,8 @@
 
 #include <asm/entry-common.h>
 
+#include <trace/events/context_tracking.h>
+
 /*
  * Define dummy _TIF work flags if not defined by the architecture or for
  * disabled functionality.
@@ -115,6 +117,7 @@ static __always_inline void enter_from_user_mode(struct pt_regs *regs)
 	instrumentation_begin();
 	kmsan_unpoison_entry_regs(regs);
 	trace_hardirqs_off_finish();
+	trace_user_exit(0);
 	instrumentation_end();
 }
 
@@ -357,6 +360,7 @@ static __always_inline void exit_to_user_mode_prepare(struct pt_regs *regs)
 static __always_inline void exit_to_user_mode(void)
 {
 	instrumentation_begin();
+	trace_user_enter(0);
 	trace_hardirqs_on_prepare();
 	lockdep_hardirqs_on_prepare();
 	instrumentation_end();
diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index fb5be6e9b423..e9395936bded 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -428,9 +428,6 @@ static __always_inline void ct_kernel_enter(bool user, int offset) { }
 
 #ifdef CONFIG_CONTEXT_TRACKING_USER
 
-#define CREATE_TRACE_POINTS
-#include <trace/events/context_tracking.h>
-
 DEFINE_STATIC_KEY_FALSE_RO(context_tracking_key);
 EXPORT_SYMBOL_GPL(context_tracking_key);
 
@@ -486,7 +483,6 @@ void noinstr __ct_user_enter(enum ctx_state state)
 			 */
 			if (state == CT_STATE_USER) {
 				instrumentation_begin();
-				trace_user_enter(0);
 				vtime_user_enter(current);
 				instrumentation_end();
 			}
@@ -623,7 +619,6 @@ void noinstr __ct_user_exit(enum ctx_state state)
 			if (state == CT_STATE_USER) {
 				instrumentation_begin();
 				vtime_user_exit(current);
-				trace_user_exit(0);
 				instrumentation_end();
 			}
 





-- 
Frederic Weisbecker
SUSE Labs