[Buildroot] [PATCH] package/musl: Make scheduler functions Linux-compatible

[Arnout Vandecappelle]

On 14/05/2019 19:31, stefan.nickl at gmail.com wrote:
> The POSIX functions sched_getscheduler(), sched_setscheduler(),
> sched_getparam(), sched_setparam() are technically not correctly
> implemented by the Linux syscalls of the same name, because what the
> kernel calls a PID and what POSIX calls a PID isn't truly the same,
> resulting in somewhat different semantics as to what these functions
> exactly apply to.
> Details: https://sourceware.org/bugzilla/show_bug.cgi?id=14829
> 
> Since the musl developers put a high premium on POSIX compliance, they
> deliberately implement these functions to return -ENOSYS instead of
> relaying them to the respective Linux syscalls as glibc/uClibc do.
> 
> Unfortunally this breaks virtually all Linux programs using these
> functions under musl. For example running 'chrt -p 1' fails with
> 'Function not implemented' on a musl-libc based system.
> In particular, it affects embedded systems using these interfaces
> for scheduling real-time processes.
> 
> As it seems unfeasible to fix all affected programs to manually use
> syscall wrappers instead of the libc functions, make musl behave the
> Linux way.

 As Thomas wrote, the patch as it is is not upstreamable since musl considers
this a feature.

 However, it is possible that Rich can be convinced to actually read the POSIX
spec and decide that linux behaviour is correct (except for the fact that it
sched_setscheduler() should return the previous scheduling policy rather than 0,
but that can be approximated (in a racy way) by calling sched_getscheduler()
first). So it might be worth trying to push the patch upstream.

> Signed-off-by: Stefan Nickl <Stefan.Nickl at gmail.com>
> ---
>  ...e-scheduler-functions-Linux-compatib.patch | 76 +++++++++++++++++++
>  1 file changed, 76 insertions(+)
>  create mode 100644 package/musl/0002-package-musl-Make-scheduler-functions-Linux-compatib.patch
> 
> diff --git a/package/musl/0002-package-musl-Make-scheduler-functions-Linux-compatib.patch b/package/musl/0002-package-musl-Make-scheduler-functions-Linux-compatib.patch
> new file mode 100644
> index 0000000000..7c3acf9f02
> --- /dev/null
> +++ b/package/musl/0002-package-musl-Make-scheduler-functions-Linux-compatib.patch
> @@ -0,0 +1,76 @@
> +From 407c96fc790d0d11ca9603a2a533216c745b5051 Mon Sep 17 00:00:00 2001
> +From: Stefan Nickl <Stefan.Nickl at gmail.com>
> +Date: Mon, 13 May 2019 22:33:21 +0200
> +Subject: [PATCH] Make scheduler functions Linux-compatible
> +
> +Let sched_getscheduler(), sched_setscheduler(), sched_getparam(),
> +sched_setparam() invoke the Linux syscalls of the same name instead
> +of returning -ENOSYS.

 ... but then you'd need to refer to
https://pubs.opengroup.org/onlinepubs/7908799/xsh/sched_setparam.html
and explain that in Linux, a pid_t can only refer to a process (because a
thread's PID can only be obtained with gettid() or clone(), neither of which is
available in musl), and that POSIX specifies that in a process with multiple
threads, sched_setscheduler should affect only the main thread, which is exactly
what Linux does.

 Either way (whether you upstream it or not), this patch looks good to me.

Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout at mind.be>

 Regards,
 Arnout

> +
> +Signed-off-by: Stefan Nickl <Stefan.Nickl at gmail.com>
> +---
> + src/sched/sched_getparam.c     | 3 +--
> + src/sched/sched_getscheduler.c | 3 +--
> + src/sched/sched_setparam.c     | 3 +--
> + src/sched/sched_setscheduler.c | 3 +--
> + 4 files changed, 4 insertions(+), 8 deletions(-)
> +
> +diff --git a/src/sched/sched_getparam.c b/src/sched/sched_getparam.c
> +index 76f10e4..65be107 100644
> +--- a/src/sched/sched_getparam.c
> ++++ b/src/sched/sched_getparam.c
> +@@ -1,8 +1,7 @@
> + #include <sched.h>
> +-#include <errno.h>
> + #include "syscall.h"
> + 
> + int sched_getparam(pid_t pid, struct sched_param *param)
> + {
> +-	return __syscall_ret(-ENOSYS);
> ++	return syscall(SYS_sched_getparam, pid, param);
> + }
> +diff --git a/src/sched/sched_getscheduler.c b/src/sched/sched_getscheduler.c
> +index 394e508..4c922f6 100644
> +--- a/src/sched/sched_getscheduler.c
> ++++ b/src/sched/sched_getscheduler.c
> +@@ -1,8 +1,7 @@
> + #include <sched.h>
> +-#include <errno.h>
> + #include "syscall.h"
> + 
> + int sched_getscheduler(pid_t pid)
> + {
> +-	return __syscall_ret(-ENOSYS);
> ++	return syscall(SYS_sched_getscheduler, pid);
> + }
> +diff --git a/src/sched/sched_setparam.c b/src/sched/sched_setparam.c
> +index 18623ee..f699faf 100644
> +--- a/src/sched/sched_setparam.c
> ++++ b/src/sched/sched_setparam.c
> +@@ -1,8 +1,7 @@
> + #include <sched.h>
> +-#include <errno.h>
> + #include "syscall.h"
> + 
> + int sched_setparam(pid_t pid, const struct sched_param *param)
> + {
> +-	return __syscall_ret(-ENOSYS);
> ++	return syscall(SYS_sched_setparam, pid, param);
> + }
> +diff --git a/src/sched/sched_setscheduler.c b/src/sched/sched_setscheduler.c
> +index 4435f21..e678221 100644
> +--- a/src/sched/sched_setscheduler.c
> ++++ b/src/sched/sched_setscheduler.c
> +@@ -1,8 +1,7 @@
> + #include <sched.h>
> +-#include <errno.h>
> + #include "syscall.h"
> + 
> + int sched_setscheduler(pid_t pid, int sched, const struct sched_param *param)
> + {
> +-	return __syscall_ret(-ENOSYS);
> ++	return syscall(SYS_sched_setscheduler, pid, sched, param);
> + }
> +-- 
> +2.21.0
> +
>