[PATCH v4 3/3] chrt: Make priority optional for policies that don't use it

[Madadi Vineeth Reddy <vineethr@linux.ibm.com>]

Currently, chrt requires a priority argument even for scheduling
policies like SCHED_OTHER and SCHED_BATCH, which ignore it.

This change relaxes that requirement. Now, priority is only expected
for SCHED_FIFO and SCHED_RR. For other policies, a default value of 0
is set internally and no argument is required on the command line.

This simplifies usage when modifying runtime parameters like
--sched-runtime for non-realtime tasks.

For example, to change the EEVDF tunable base_slice, one currently
needs to run:
chrt -o -T 1000000 --pid 0 $PID

Passing '0' after --pid is not intutive and not required as priority
is not applicable to SCHED_OTHER tasks. Now with this patch, one can do:
chrt -o -T 1000000 --pid $PID

Passing '0' still works ensuring ABI doesn't break.

Signed-off-by: Madadi Vineeth Reddy <vineethr@linux.ibm.com>
---
 schedutils/chrt.1.adoc | 14 +++++++++-----
 schedutils/chrt.c      | 24 ++++++++++++++++--------
 2 files changed, 25 insertions(+), 13 deletions(-)

diff --git a/schedutils/chrt.1.adoc b/schedutils/chrt.1.adoc
index 36cdcc5fe..5b8d7e832 100644
--- a/schedutils/chrt.1.adoc
+++ b/schedutils/chrt.1.adoc
@@ -49,7 +49,7 @@ chrt - manipulate the real-time attributes of a process
 == POLICIES
 
 *-o*, *--other*::
-Set scheduling policy to *SCHED_OTHER* (time-sharing scheduling). This is the default Linux scheduling policy.
+Set scheduling policy to *SCHED_OTHER* (time-sharing scheduling). This is the default Linux scheduling policy. Since util-linux v2.42, the priority argument is optional; if specified, it must be set to zero.
 
 *-f*, *--fifo*::
 Set scheduling policy to *SCHED_FIFO* (first in-first out).
@@ -58,16 +58,16 @@ Set scheduling policy to *SCHED_FIFO* (first in-first out).
 Set scheduling policy to *SCHED_RR* (round-robin scheduling). When no policy is defined, the *SCHED_RR* is used as the default.
 
 *-b*, *--batch*::
-Set scheduling policy to *SCHED_BATCH* (scheduling batch processes). Linux-specific, supported since 2.6.16. The priority argument has to be set to zero.
+Set scheduling policy to *SCHED_BATCH* (scheduling batch processes). Linux-specific, supported since 2.6.16. Since util-linux v2.42, the priority argument is optional; if specified, it must be set to zero.
 
 *-i*, *--idle*::
-Set scheduling policy to *SCHED_IDLE* (scheduling very low priority jobs). Linux-specific, supported since 2.6.23. The priority argument has to be set to zero.
+Set scheduling policy to *SCHED_IDLE* (scheduling very low priority jobs). Linux-specific, supported since 2.6.23. Since util-linux v2.42, the priority argument is optional; if specified, it must be set to zero.
 
 *-d*, *--deadline*::
-Set scheduling policy to *SCHED_DEADLINE* (sporadic task model deadline scheduling). Linux-specific, supported since 3.14. The priority argument has to be set to zero. See also *--sched-runtime*, *--sched-deadline* and *--sched-period*. The relation between the options required by the kernel is runtime <= deadline <= period. *chrt* copies _period_ to _deadline_ if *--sched-deadline* is not specified and _deadline_ to _runtime_ if *--sched-runtime* is not specified. It means that at least *--sched-period* has to be specified. See *sched*(7) for more details.
+Set scheduling policy to *SCHED_DEADLINE* (sporadic task model deadline scheduling). Linux-specific, supported since 3.14. Since util-linux v2.42, the priority argument is optional; if specified, it must be set to zero. See also *--sched-runtime*, *--sched-deadline* and *--sched-period*. The relation between the options required by the kernel is runtime <= deadline <= period. *chrt* copies _period_ to _deadline_ if *--sched-deadline* is not specified and _deadline_ to _runtime_ if *--sched-runtime* is not specified. It means that at least *--sched-period* has to be specified. See *sched*(7) for more details.
 
 *-d*, *--ext*::
-Set scheduling policy to *SCHED_EXT* (BPF program-defined scheduling). Linux-specific, supported since 6.12. The priority argument has to be set to zero.
+Set scheduling policy to *SCHED_EXT* (BPF program-defined scheduling). Linux-specific, supported since 6.12. Since util-linux v2.42, the priority argument is optional; if specified, it must be set to zero.
 
 == SCHEDULING OPTIONS
 
@@ -132,6 +132,10 @@ Reset priorities to default for a process{colon}::
 ____
 *chrt -o --pid 0* _PID_
 ____
+Set a custom slice of 1 ms for a SCHED_OTHER task (priority is optional for policies other than SCHED_FIFO and SCHED_RR){colon}::
+____
+*chrt -o -T 1000000 --pid* _PID_
+____
 See *sched*(7) for a detailed discussion of the different scheduler classes and how they interact.
 
 == PERMISSIONS
diff --git a/schedutils/chrt.c b/schedutils/chrt.c
index d694845ac..0bcdd1a1e 100644
--- a/schedutils/chrt.c
+++ b/schedutils/chrt.c
@@ -399,7 +399,7 @@ int main(int argc, char **argv)
 {
 	struct chrt_ctl _ctl = { .pid = -1, .policy = SCHED_RR }, *ctl = &_ctl;
 	int c;
-	bool policy_given = false;
+	bool policy_given = false, need_prio = false;
 
 	static const struct option longopts[] = {
 		{ "all-tasks",  no_argument, NULL, 'a' },
@@ -455,6 +455,7 @@ int main(int argc, char **argv)
 		case 'f':
 			ctl->policy = SCHED_FIFO;
 			policy_given = true;
+			need_prio = true;
 			break;
 		case 'R':
 			ctl->reset_on_fork = 1;
@@ -480,6 +481,7 @@ int main(int argc, char **argv)
 		case 'r':
 			ctl->policy = SCHED_RR;
 			policy_given = true;
+			need_prio = true;
 			break;
 		case 'v':
 			ctl->verbose = 1;
@@ -503,29 +505,35 @@ int main(int argc, char **argv)
 		}
 	}
 
-	if (((ctl->pid > -1) && argc - optind < 1) ||
-	    ((ctl->pid == -1) && argc - optind < 2)) {
+	if (((ctl->pid > -1) && argc - optind < (need_prio ? 1 : 0)) ||
+	    ((ctl->pid == -1) && argc - optind < (need_prio ? 2 : 1))) {
 		warnx(_("bad usage"));
 		errtryhelp(EXIT_FAILURE);
 	}
 
 	/* pid exists but priority not given */
 	if (ctl->pid > -1 && argc - optind == 1) {
-		/* Error if a policy was specified but no priority given */
-		if (policy_given)
+		/* Error if priority is missing for a policy that requires it */
+		if (policy_given && need_prio)
 			errx(EXIT_FAILURE, ("policy %s requires a priority argument"),
 						get_policy_name(ctl->policy));
 
 		/* If no policy specified, show current settings */
-		show_sched_info(ctl);
-		return EXIT_SUCCESS;
+		if (!policy_given) {
+			show_sched_info(ctl);
+			return EXIT_SUCCESS;
+		}
 	}
 
 	if (ctl->verbose)
 		show_sched_info(ctl);
 
 	errno = 0;
-	ctl->priority = strtos32_or_err(argv[optind], _("invalid priority argument"));
+
+	if (need_prio || argc - optind == 2)
+		ctl->priority = strtos32_or_err(argv[optind], _("invalid priority argument"));
+	else
+		ctl->priority = 0;
 
 	if (ctl->runtime && !supports_runtime_param(ctl->policy))
 		errx(EXIT_FAILURE, _("--sched-runtime option is supported for %s"),
-- 
2.49.0