[PATCH] 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 -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 <vineethr@linux.ibm.com>
---
 schedutils/chrt.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/schedutils/chrt.c b/schedutils/chrt.c
index cf99935dc..339f3e318 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 policy_needs_priority = (ctl->policy == SCHED_FIFO || ctl->policy == SCHED_RR);
+
+	if (((ctl->pid > -1) && argc - optind < (policy_needs_priority ? 1 : 0)) ||
+	    ((ctl->pid == -1) && argc - optind < (policy_needs_priority ? 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 == (policy_needs_priority ? 1 : 0))) {
 		show_sched_info(ctl);
-		if (argc - optind == 1)
+		if (argc - optind == (policy_needs_priority ? 1 : 0))
 			return EXIT_SUCCESS;
 	}
 
 	errno = 0;
-	ctl->priority = strtos32_or_err(argv[optind], _("invalid priority argument"));
+
+	if (policy_needs_priority || 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