rtla/timerlat: Add --dma-latency option

Add the --dma-latency to set /dev/cpu_dma_latency to the
specified value, this aims to avoid having exit from idle
states latencies that could be influencing the analysis.

Link: https://lkml.kernel.org/r/72ddb0d913459f13217086dadafad88a7c46dd28.1646247211.git.bristot@kernel.org

Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
Cc: Clark Williams <williams@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: Daniel Bristot de Oliveira

Documentation/tools/rtla/common_timerlat_options.rst

@@ -21,3 +21,8 @@
 
         Save the stack trace at the *IRQ* if a *Thread* latency is higher than the
         argument in us.
+
+**--dma-latency** *us*
+        Set the /dev/cpu_dma_latency to *us*, aiming to bound exit from idle latencies.
+        *cyclictest* sets this value to *0* by default, use **--dma-latency** *0* to have
+        similar results.

tools/tracing/rtla/src/timerlat_hist.c

@@ -28,6 +28,7 @@ struct timerlat_hist_params {
 	int			output_divisor;
 	int			duration;
 	int			set_sched;
+	int			dma_latency;
 	struct sched_attr	sched_param;
 	struct trace_events	*events;
 
@@ -432,7 +433,7 @@ static void timerlat_hist_usage(char *usage)
 		"  usage: [rtla] timerlat hist [-h] [-q] [-d s] [-D] [-n] [-a us] [-p us] [-i us] [-T us] [-s us] \\",
 		"         [-t[=file]] [-e sys[:event]] [--filter <filter>] [--trigger <trigger>] [-c cpu-list] \\",
 		"	  [-P priority] [-E N] [-b N] [--no-irq] [--no-thread] [--no-header] [--no-summary] \\",
-		"	  [--no-index] [--with-zeros]",
+		"	  [--no-index] [--with-zeros] [--dma-latency us]",
 		"",
 		"	  -h/--help: print this menu",
 		"	  -a/--auto: set automatic trace mode, stopping the session if argument in us latency is hit",
@@ -456,6 +457,7 @@ static void timerlat_hist_usage(char *usage)
 		"	     --no-summary: do not print summary",
 		"	     --no-index: do not print index",
 		"	     --with-zeros: print zero only entries",
+		"	     --dma-latency us: set /dev/cpu_dma_latency latency <us> to reduce exit from idle latency",
 		"	  -P/--priority o:prio|r:prio|f:prio|d:runtime:period : set scheduling parameters",
 		"		o:prio - use SCHED_OTHER with prio",
 		"		r:prio - use SCHED_RR with prio",
@@ -492,6 +494,9 @@ static struct timerlat_hist_params
 	if (!params)
 		exit(1);
 
+	/* disabled by default */
+	params->dma_latency = -1;
+
 	/* display data in microseconds */
 	params->output_divisor = 1000;
 	params->bucket_size = 1;
@@ -522,13 +527,14 @@ static struct timerlat_hist_params
 			{"with-zeros",		no_argument,		0, '5'},
 			{"trigger",		required_argument,	0, '6'},
 			{"filter",		required_argument,	0, '7'},
+			{"dma-latency",		required_argument,	0, '8'},
 			{0, 0, 0, 0}
 		};
 
 		/* getopt_long stores the option index here. */
 		int option_index = 0;
 
-		c = getopt_long(argc, argv, "a:c:b:d:e:E:Dhi:np:P:s:t::T:0123456:7:",
+		c = getopt_long(argc, argv, "a:c:b:d:e:E:Dhi:np:P:s:t::T:0123456:7:8:",
 				 long_options, &option_index);
 
 		/* detect the end of the options. */
@@ -659,6 +665,13 @@ static struct timerlat_hist_params
 				timerlat_hist_usage("--filter requires a previous -e\n");
 			}
 			break;
+		case '8':
+			params->dma_latency = get_llong_from_str(optarg);
+			if (params->dma_latency < 0 || params->dma_latency > 10000) {
+				err_msg("--dma-latency needs to be >= 0 and < 10000");
+				exit(EXIT_FAILURE);
+			}
+			break;
 		default:
 			timerlat_hist_usage("Invalid option");
 		}
@@ -791,6 +804,7 @@ int timerlat_hist_main(int argc, char *argv[])
 	struct osnoise_tool *record = NULL;
 	struct osnoise_tool *tool = NULL;
 	struct trace_instance *trace;
+	int dma_latency_fd = -1;
 	int return_value = 1;
 	int retval;
 
@@ -826,6 +840,14 @@ int timerlat_hist_main(int argc, char *argv[])
 		}
 	}
 
+	if (params->dma_latency >= 0) {
+		dma_latency_fd = set_cpu_dma_latency(params->dma_latency);
+		if (dma_latency_fd < 0) {
+			err_msg("Could not set /dev/cpu_dma_latency.\n");
+			goto out_hist;
+		}
+	}
+
 	trace_instance_start(trace);
 
 	if (params->trace_output) {
@@ -878,6 +900,8 @@ int timerlat_hist_main(int argc, char *argv[])
 	}
 
 out_hist:
+	if (dma_latency_fd >= 0)
+		close(dma_latency_fd);
 	trace_events_destroy(&record->trace, params->events);
 	params->events = NULL;
 	timerlat_free_histogram(tool->data);

tools/tracing/rtla/src/timerlat_top.c

@@ -29,6 +29,7 @@ struct timerlat_top_params {
 	int			duration;
 	int			quiet;
 	int			set_sched;
+	int			dma_latency;
 	struct sched_attr	sched_param;
 	struct trace_events	*events;
 };
@@ -269,7 +270,7 @@ static void timerlat_top_usage(char *usage)
 		"",
 		"  usage: rtla timerlat [top] [-h] [-q] [-a us] [-d s] [-D] [-n] [-p us] [-i us] [-T us] [-s us] \\",
 		"	  [[-t[=file]] [-e sys[:event]] [--filter <filter>] [--trigger <trigger>] [-c cpu-list] \\",
-		"	  [-P priority]",
+		"	  [-P priority] [--dma-latency us]",
 		"",
 		"	  -h/--help: print this menu",
 		"	  -a/--auto: set automatic trace mode, stopping the session if argument in us latency is hit",
@@ -286,6 +287,7 @@ static void timerlat_top_usage(char *usage)
 		"	     --trigger <command>: enable a trace event trigger to the previous -e event",
 		"	  -n/--nano: display data in nanoseconds",
 		"	  -q/--quiet print only a summary at the end",
+		"	     --dma-latency us: set /dev/cpu_dma_latency latency <us> to reduce exit from idle latency",
 		"	  -P/--priority o:prio|r:prio|f:prio|d:runtime:period : set scheduling parameters",
 		"		o:prio - use SCHED_OTHER with prio",
 		"		r:prio - use SCHED_RR with prio",
@@ -322,6 +324,9 @@ static struct timerlat_top_params
 	if (!params)
 		exit(1);
 
+	/* disabled by default */
+	params->dma_latency = -1;
+
 	/* display data in microseconds */
 	params->output_divisor = 1000;
 
@@ -343,13 +348,14 @@ static struct timerlat_top_params
 			{"trace",		optional_argument,	0, 't'},
 			{"trigger",		required_argument,	0, '0'},
 			{"filter",		required_argument,	0, '1'},
+			{"dma-latency",		required_argument,	0, '2'},
 			{0, 0, 0, 0}
 		};
 
 		/* getopt_long stores the option index here. */
 		int option_index = 0;
 
-		c = getopt_long(argc, argv, "a:c:d:De:hi:np:P:qs:t::T:0:1:",
+		c = getopt_long(argc, argv, "a:c:d:De:hi:np:P:qs:t::T:0:1:2:",
 				 long_options, &option_index);
 
 		/* detect the end of the options. */
@@ -454,6 +460,13 @@ static struct timerlat_top_params
 				timerlat_top_usage("--filter requires a previous -e\n");
 			}
 			break;
+		case '2': /* dma-latency */
+			params->dma_latency = get_llong_from_str(optarg);
+			if (params->dma_latency < 0 || params->dma_latency > 10000) {
+				err_msg("--dma-latency needs to be >= 0 and < 10000");
+				exit(EXIT_FAILURE);
+			}
+			break;
 		default:
 			timerlat_top_usage("Invalid option");
 		}
@@ -582,6 +595,7 @@ int timerlat_top_main(int argc, char *argv[])
 	struct osnoise_tool *record = NULL;
 	struct osnoise_tool *top = NULL;
 	struct trace_instance *trace;
+	int dma_latency_fd = -1;
 	int return_value = 1;
 	int retval;
 
@@ -617,6 +631,14 @@ int timerlat_top_main(int argc, char *argv[])
 		}
 	}
 
+	if (params->dma_latency >= 0) {
+		dma_latency_fd = set_cpu_dma_latency(params->dma_latency);
+		if (dma_latency_fd < 0) {
+			err_msg("Could not set /dev/cpu_dma_latency.\n");
+			goto out_top;
+		}
+	}
+
 	trace_instance_start(trace);
 
 	if (params->trace_output) {
@@ -673,6 +695,8 @@ int timerlat_top_main(int argc, char *argv[])
 	}
 
 out_top:
+	if (dma_latency_fd >= 0)
+		close(dma_latency_fd);
 	trace_events_destroy(&record->trace, params->events);
 	params->events = NULL;
 	timerlat_free_top(top->data);

tools/tracing/rtla/src/utils.c

@@ -10,6 +10,7 @@
 #include <unistd.h>
 #include <ctype.h>
 #include <errno.h>
+#include <fcntl.h>
 #include <sched.h>
 #include <stdio.h>
 
@@ -431,3 +432,35 @@ int parse_prio(char *arg, struct sched_attr *sched_param)
 	}
 	return 0;
 }
+
+/*
+ * set_cpu_dma_latency - set the /dev/cpu_dma_latecy
+ *
+ * This is used to reduce the exit from idle latency. The value
+ * will be reset once the file descriptor of /dev/cpu_dma_latecy
+ * is closed.
+ *
+ * Return: the /dev/cpu_dma_latecy file descriptor
+ */
+int set_cpu_dma_latency(int32_t latency)
+{
+	int retval;
+	int fd;
+
+	fd = open("/dev/cpu_dma_latency", O_RDWR);
+	if (fd < 0) {
+		err_msg("Error opening /dev/cpu_dma_latency\n");
+		return -1;
+	}
+
+	retval = write(fd, &latency, 4);
+	if (retval < 1) {
+		err_msg("Error setting /dev/cpu_dma_latency\n");
+		close(fd);
+		return -1;
+	}
+
+	debug_msg("Set /dev/cpu_dma_latency to %d\n", latency);
+
+	return fd;
+}

tools/tracing/rtla/src/utils.h

@@ -54,3 +54,4 @@ struct sched_attr {
 
 int parse_prio(char *arg, struct sched_attr *sched_param);
 int set_comm_sched_attr(const char *comm, struct sched_attr *attr);
+int set_cpu_dma_latency(int32_t latency);