selftests/resctrl: Only support measured read operation

The CMT, MBM, and MBA tests rely on a benchmark to generate
memory traffic. By default this is the "fill_buf" benchmark that
can be replaced via the "-b" command line argument.

The original intent of the "-b" command line parameter was
to replace the default "fill_buf" benchmark, but the implementation
also exposes an alternative use case where the "fill_buf" parameters
itself can be modified. One of the parameters to "fill_buf" is the
"operation" that can be either "read" or "write" and indicates
whether the "fill_buf" should use "read" or "write" operations on the
allocated buffer.

While replacing "fill_buf" default parameters is technically possible,
replacing the default "read" parameter with "write" is not supported
because the MBA and MBM tests only measure "read" operations. The
"read" operation is also most appropriate for the CMT test that aims
to use the benchmark to allocate into the cache.

Avoid any potential inconsistencies between test and measurement by
removing code for unsupported "write" operations to the buffer.
Ignore any attempt from user space to enable this unsupported test
configuration, instead always use read operations.

Keep the initialization of the, now unused, "fill_buf" parameters
to reserve these parameter positions since it has been exposed as an API.
Future parameter additions cannot use these parameter positions.

Signed-off-by: Reinette Chatre <[e-mail]>
Reviewed-by: Ilpo Järvinen <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/resctrl/fill_buf.c

···
 	return sum;
 }
 
-static void fill_one_span_write(unsigned char *buf, size_t buf_size)
-{
-	unsigned char *end_ptr = buf + buf_size;
-	unsigned char *p;
-
-	p = buf;
-	while (p < end_ptr) {
-		*p = '1';
-		p += (CL_SIZE / 2);
-	}
-}
-
 void fill_cache_read(unsigned char *buf, size_t buf_size, bool once)
 {
 	int ret = 0;
···
 
 	/* Consume read result so that reading memory is not optimized out. */
 	*value_sink = ret;
-}
-
-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;
-	}
 }
 
 unsigned char *alloc_buffer(size_t buf_size, int memflush)
···
 	return buf;
 }
 
-int run_fill_buf(size_t buf_size, int memflush, int op)
+int run_fill_buf(size_t buf_size, int memflush)
 {
 	unsigned char *buf;
 
···
 	if (!buf)
 		return -1;
 
-	if (op == 0)
-		fill_cache_read(buf, buf_size, false);
-	else
-		fill_cache_write(buf, buf_size, false);
+	fill_cache_read(buf, buf_size, false);
 
 	free(buf);
 

tools/testing/selftests/resctrl/resctrl.h

···
 unsigned char *alloc_buffer(size_t buf_size, int memflush);
 void mem_flush(unsigned char *buf, size_t buf_size);
 void fill_cache_read(unsigned char *buf, size_t buf_size, bool once);
-int run_fill_buf(size_t buf_size, int memflush, int op);
+int run_fill_buf(size_t buf_size, int memflush);
 int initialize_mem_bw_imc(void);
 int measure_mem_bw(const struct user_params *uparams,
 		   struct resctrl_val_param *param, pid_t bm_pid,

tools/testing/selftests/resctrl/resctrl_tests.c

···
 			ksft_exit_fail_msg("Out of memory!\n");
 		uparams.benchmark_cmd[1] = span_str;
 		uparams.benchmark_cmd[2] = "1";
-		uparams.benchmark_cmd[3] = "0";
 		/*
+		 * Third parameter was previously used for "operation"
+		 * (read/write) of which only (now default) "read"/"0"
+		 * works.
 		 * Fourth parameter was previously used to indicate
 		 * how long "fill_buf" should run for, with "false"
 		 * ("fill_buf" will keep running until terminated)
 		 * the only option that works.
 		 */
+		uparams.benchmark_cmd[3] = NULL;
 		uparams.benchmark_cmd[4] = NULL;
 	}
 

tools/testing/selftests/resctrl/resctrl_val.c

···
  */
 static void run_benchmark(int signum, siginfo_t *info, void *ucontext)
 {
-	int operation, ret, memflush;
 	char **benchmark_cmd;
+	int ret, memflush;
 	size_t span;
 	FILE *fp;
 
···
 		/* Execute default fill_buf benchmark */
 		span = strtoul(benchmark_cmd[1], NULL, 10);
 		memflush =  atoi(benchmark_cmd[2]);
-		operation = atoi(benchmark_cmd[3]);
 
-		if (run_fill_buf(span, memflush, operation))
+		if (run_fill_buf(span, memflush))
 			fprintf(stderr, "Error in running fill buffer\n");
 	} else {
 		/* Execute specified benchmark */