diff mbox series

[v2,17/26] selftests/resctrl: Replace file write with volatile variable

Message ID 20231120111340.7805-18-ilpo.jarvinen@linux.intel.com
State Superseded
Headers show
Series [v2,01/26] selftests/resctrl: Don't use ctrlc_handler() outside signal handling | expand

Commit Message

Ilpo Järvinen Nov. 20, 2023, 11:13 a.m. UTC
The fill_buf code prevents compiler optimizating the entire read loop
away by writing the final value of the variable into a file. While it
achieves the goal, writing into a file requires significant amount of
work within the innermost test loop and also error handling.

A simpler approach is to take advantage of volatile. Writing to a
variable through a volatile pointer is enough to prevent compiler from
optimizing the write away, and therefore compiler cannot remove the
read loop either.

Add a volatile 'value_sink' into resctrl_tests.c and make fill_buf to
write into it. As a result, the error handling in fill_buf.c can be
simplified.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
---
 tools/testing/selftests/resctrl/fill_buf.c    | 26 ++++---------------
 tools/testing/selftests/resctrl/resctrl.h     |  7 +++++
 .../testing/selftests/resctrl/resctrl_tests.c |  4 +++
 3 files changed, 16 insertions(+), 21 deletions(-)

Comments

Reinette Chatre Nov. 28, 2023, 10:16 p.m. UTC | #1
Hi Ilpo,

On 11/20/2023 3:13 AM, Ilpo Järvinen wrote:
> The fill_buf code prevents compiler optimizating the entire read loop
> away by writing the final value of the variable into a file. While it
> achieves the goal, writing into a file requires significant amount of
> work within the innermost test loop and also error handling.
> 
> A simpler approach is to take advantage of volatile. Writing to a
> variable through a volatile pointer is enough to prevent compiler from
> optimizing the write away, and therefore compiler cannot remove the
> read loop either.
> 
> Add a volatile 'value_sink' into resctrl_tests.c and make fill_buf to
> write into it. As a result, the error handling in fill_buf.c can be
> simplified.
> 

The subject and changelog describes the need for a volatile variable.
The patch introduces two volatile variables. Could you please elaborate
why two volatile variables are needed?

Reinette
Ilpo Järvinen Dec. 7, 2023, 3:03 p.m. UTC | #2
On Tue, 28 Nov 2023, Reinette Chatre wrote:
> On 11/20/2023 3:13 AM, Ilpo Järvinen wrote:
> > The fill_buf code prevents compiler optimizating the entire read loop
> > away by writing the final value of the variable into a file. While it
> > achieves the goal, writing into a file requires significant amount of
> > work within the innermost test loop and also error handling.
> > 
> > A simpler approach is to take advantage of volatile. Writing to a
> > variable through a volatile pointer is enough to prevent compiler from
> > optimizing the write away, and therefore compiler cannot remove the
> > read loop either.
> > 
> > Add a volatile 'value_sink' into resctrl_tests.c and make fill_buf to
> > write into it. As a result, the error handling in fill_buf.c can be
> > simplified.
> > 
> 
> The subject and changelog describes the need for a volatile variable.
> The patch introduces two volatile variables. Could you please elaborate
> why two volatile variables are needed?

Well, the other "volatile variable" is a pointer to a volatile variable.

I've seen gcc to kill a static volatile int so I prefer to not take 
change with its optimizer. Thus, I placed the sink into different 
compilation unit and just present a pointer to the actual "volatile" 
variable.

I guess the sink could be marked as plain int instead but this being 
trickery to begin with I don't see much value either way. It's still
a trick.

I'll alter the changelog's wording though, "a volatile variable" is
not accurate as it's "a pointer to a volatile variable".
diff mbox series

Patch

diff --git a/tools/testing/selftests/resctrl/fill_buf.c b/tools/testing/selftests/resctrl/fill_buf.c
index b9303a9d819b..8fe9574db9d8 100644
--- a/tools/testing/selftests/resctrl/fill_buf.c
+++ b/tools/testing/selftests/resctrl/fill_buf.c
@@ -78,10 +78,9 @@  static void fill_one_span_write(unsigned char *buf, size_t buf_size)
 	}
 }
 
-static int fill_cache_read(unsigned char *buf, size_t buf_size, bool once)
+static void fill_cache_read(unsigned char *buf, size_t buf_size, bool once)
 {
 	int ret = 0;
-	FILE *fp;
 
 	while (1) {
 		ret = fill_one_span_read(buf, buf_size);
@@ -90,26 +89,16 @@  static int fill_cache_read(unsigned char *buf, size_t buf_size, bool once)
 	}
 
 	/* Consume read result so that reading memory is not optimized out. */
-	fp = fopen("/dev/null", "w");
-	if (!fp) {
-		perror("Unable to write to /dev/null");
-		return -1;
-	}
-	fprintf(fp, "Sum: %d ", ret);
-	fclose(fp);
-
-	return 0;
+	*value_sink = ret;
 }
 
-static int fill_cache_write(unsigned char *buf, size_t buf_size, bool once)
+static void fill_cache_write(unsigned char *buf, size_t buf_size, bool once)
 {
 	while (1) {
 		fill_one_span_write(buf, buf_size);
 		if (once)
 			break;
 	}
-
-	return 0;
 }
 
 static unsigned char *alloc_buffer(size_t buf_size, int memflush)
@@ -143,21 +132,16 @@  static unsigned char *alloc_buffer(size_t buf_size, int memflush)
 int run_fill_buf(size_t buf_size, int memflush, int op, bool once)
 {
 	unsigned char *buf;
-	int ret;
 
 	buf = alloc_buffer(buf_size, memflush);
 	if (!buf)
 		return -1;
 
 	if (op == 0)
-		ret = fill_cache_read(buf, buf_size, once);
+		fill_cache_read(buf, buf_size, once);
 	else
-		ret = fill_cache_write(buf, buf_size, once);
+		fill_cache_write(buf, buf_size, once);
 	free(buf);
-	if (ret) {
-		printf("\n Error in fill cache\n");
-		return -1;
-	}
 
 	return 0;
 }
diff --git a/tools/testing/selftests/resctrl/resctrl.h b/tools/testing/selftests/resctrl/resctrl.h
index fe07b732cbd9..68eb1f1bac3a 100644
--- a/tools/testing/selftests/resctrl/resctrl.h
+++ b/tools/testing/selftests/resctrl/resctrl.h
@@ -79,6 +79,13 @@  struct perf_event_read {
 #define CMT_STR			"cmt"
 #define CAT_STR			"cat"
 
+/*
+ * Memory location that consumes values compiler must not optimize away.
+ * Volatile ensures writes to this location cannot be optimized away by
+ * compiler.
+ */
+extern volatile int *value_sink;
+
 extern pid_t bm_pid, ppid;
 
 extern char llc_occup_path[1024];
diff --git a/tools/testing/selftests/resctrl/resctrl_tests.c b/tools/testing/selftests/resctrl/resctrl_tests.c
index 2bbe3045a018..f0d573aa363b 100644
--- a/tools/testing/selftests/resctrl/resctrl_tests.c
+++ b/tools/testing/selftests/resctrl/resctrl_tests.c
@@ -10,6 +10,10 @@ 
  */
 #include "resctrl.h"
 
+/* Volatile memory sink to prevent compiler optimizations */
+static volatile int sink_target;
+volatile int *value_sink = &sink_target;
+
 static int detect_vendor(void)
 {
 	FILE *inf = fopen("/proc/cpuinfo", "r");