[PATCH v2] chrt: Make priority optional for policies that don't use it

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 -v -o -T 1000000 -p 0 $PID

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

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

Signed-off-by: Madadi Vineeth Reddy <[email protected]>
---
Changes in v2:
- Updated the man page to reflect the optional priority behavior (Karel Zak)
- Renamed variable to 'need_prio' (Karel Zak)

Signed-off-by: Madadi Vineeth Reddy <[email protected]>
---
 schedutils/chrt.1.adoc | 12 ++++++++----
 schedutils/chrt.c      | 16 +++++++++++-----
 2 files changed, 19 insertions(+), 9 deletions(-)

diff --git a/schedutils/chrt.1.adoc b/schedutils/chrt.1.adoc
index 77add535d..4f419b5f6 100644
--- a/schedutils/chrt.1.adoc
+++ b/schedutils/chrt.1.adoc
@@ -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 -p 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 -p* _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 cf99935dc..eb1717acc 100644
--- a/schedutils/chrt.c
+++ b/schedutils/chrt.c
@@ -495,20 +495,26 @@ int main(int argc, char **argv)
 		}
 	}
 
-	if (((ctl->pid > -1) && argc - optind < 1) ||
-	    ((ctl->pid == -1) && argc - optind < 2)) {
+	bool need_prio = (ctl->policy == SCHED_FIFO || ctl->policy == SCHED_RR);
+
+	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);
 }
 
-	if ((ctl->pid > -1) && (ctl->verbose || argc - optind == 1)) {
+	if ((ctl->pid > -1) && (ctl->verbose || argc - optind == (need_prio ? 1 : 0))) {
 		show_sched_info(ctl);
-		if (argc - optind == 1)
+		if (argc - optind == (need_prio ? 1 : 0))
 			return EXIT_SUCCESS;
 	}
 
 	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