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

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

[Madadi Vineeth Reddy]

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

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

[Karel Zak]

On Mon, Jun 16, 2025 at 12:52:18AM +0530, Madadi Vineeth Reddy wrote:
> 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.

Looks good. It would be nice to update the man page and add a note  
that -p is not required since util-linux v2.42.

> +	bool policy_needs_priority = (ctl->policy == SCHED_FIFO || ctl->policy == SCHED_RR);

Nitpicking... can't we use a shorter name, for example "need_prio"? ;-)

    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com

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

[Madadi Vineeth Reddy]

Hi Karel,

On 17/06/25 17:35, Karel Zak wrote:
> On Mon, Jun 16, 2025 at 12:52:18AM +0530, Madadi Vineeth Reddy wrote:
>> 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.
> 
> Looks good. It would be nice to update the man page and add a note  
> that -p is not required since util-linux v2.42.
> 

I believe you meant that the priority argument is not required for some
scheduling policies. The -p / --pid option is still required.

I’ll update the man page accordingly.

>> +	bool policy_needs_priority = (ctl->policy == SCHED_FIFO || ctl->policy == SCHED_RR);
> 
> Nitpicking... can't we use a shorter name, for example "need_prio"? ;-)

Sure, will update it in v2. Thanks for taking a look.

Thanks,
Madadi Vineeth Reddy

> 
>     Karel
>