[PATCH] chrt: support for musl C library

[Bernhard Reutner-Fischer]

On Fri, 11 Sep 2020 17:45:38 +0200
Christian Eggers <ceggers at arri.de> wrote:

> musl "implements" several sched_xxx() functions by returning ENOSYS. As
> an alternative, either pthread_(g|s)etschedparam() or direct syscalls
> can be used.

I don't like using the syscalls directly in the way you propose below.

OTOH pulling in pthread for the pedantically correct pthread_ functions
is crude.
But then, nowadays, the Threads option is part of Base, so what exactly
is the objection from the musl folks so they stub out
sched_getscheduler?

thanks,
> 
> References: https://git.kernel.org/pub/scm/utils/util-linux/util-linux.git/commit/schedutils/chrt.c?id=fcc3078754291d2f5121797eb91b364f8e24b2f1
> References: http://git.musl-libc.org/cgit/musl/commit/src/sched/sched_setscheduler.c?id=1e21e78bf7a5c24c217446d8760be7b7188711c2
> Signed-off-by: Christian Eggers <ceggers at arri.de>
> ---
>  util-linux/chrt.c | 31 +++++++++++++++++++++++++++++--
>  1 file changed, 29 insertions(+), 2 deletions(-)
> 
> diff --git a/util-linux/chrt.c b/util-linux/chrt.c
> index 4dd78dabf..e465d7cec 100644
> --- a/util-linux/chrt.c
> +++ b/util-linux/chrt.c
> @@ -34,6 +34,9 @@
>  //usage:       "You need CAP_SYS_NICE privileges to set scheduling attributes of a process"
>  
>  #include <sched.h>
> +#if defined (__linux__)
> +# include <sys/syscall.h>
> +#endif
>  #include "libbb.h"
>  #ifndef SCHED_IDLE
>  # define SCHED_IDLE 5
> @@ -85,6 +88,7 @@ int chrt_main(int argc UNUSED_PARAM, char **argv)
>  	char *priority = priority; /* for compiler */
>  	const char *current_new;
>  	int policy = SCHED_RR;
> +	int ret;
>  
>  	opt = getopt32(argv, "^"
>  			"+" "mprfobi"
> @@ -132,7 +136,15 @@ int chrt_main(int argc UNUSED_PARAM, char **argv)
>  	if (opt & OPT_p) {
>  		int pol;
>   print_rt_info:
> +#if defined (__linux__) && defined(SYS_sched_getscheduler)
> +		/* musl libc returns ENOSYS for its sched_getscheduler library
> +		 * function, because the sched_getscheduler Linux kernel system call
> +		 * does not conform to Posix; so we use the system call directly
> +		 */
> +		pol = syscall(SYS_sched_getscheduler, pid);
> +#else
>  		pol = sched_getscheduler(pid);
> +#endif
>  		if (pol < 0)
>  			bb_perror_msg_and_die("can't %cet pid %u's policy", 'g', (int)pid);
>  #ifdef SCHED_RESET_ON_FORK
> @@ -149,7 +161,12 @@ int chrt_main(int argc UNUSED_PARAM, char **argv)
>  		printf("pid %u's %s scheduling policy: SCHED_%s\n",
>  			pid, current_new, policy_name(pol)
>  		);
> -		if (sched_getparam(pid, &sp))
> +#if defined (__linux__) && defined(SYS_sched_getparam)
> +		ret = syscall(SYS_sched_getparam, pid, &sp);
> +#else
> +		ret = sched_getparam(pid, &sp);
> +#endif
> +		if (ret)
>  			bb_perror_msg_and_die("can't get pid %u's attributes", (int)pid);
>  		printf("pid %u's %s scheduling priority: %d\n",
>  			(int)pid, current_new, sp.sched_priority
> @@ -168,7 +185,17 @@ int chrt_main(int argc UNUSED_PARAM, char **argv)
>  		sched_get_priority_min(policy), sched_get_priority_max(policy)
>  	);
>  
> -	if (sched_setscheduler(pid, policy, &sp) < 0)
> +#if defined (__linux__) && defined(SYS_sched_setscheduler)
> +	/* musl libc returns ENOSYS for its sched_setscheduler library
> +	 * function, because the sched_setscheduler Linux kernel system call
> +	 * does not conform to Posix; so we use the system call directly
> +	 */
> +	ret = syscall(SYS_sched_setscheduler, pid, policy, &sp);
> +#else
> +	ret = sched_setscheduler(pid, policy, &sp);
> +#endif
> +
> +	if (ret < 0)
>  		bb_perror_msg_and_die("can't %cet pid %u's policy", 's', (int)pid);
>  
>  	if (!argv[0]) /* "-p PRIO PID [...]" */