Provide a new syscall which has the only purpose to yield the CPU after the
kernel granted a time slice extension.

sched_yield() is not suitable for that because it unconditionally
schedules, but the end of the time slice extension is not required to
schedule when the task was already preempted. This also allows to have a
strict check for termination to catch user space invoking random syscalls
including sched_yield() from a time slice extension region.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: linux-arch@vger.kernel.org
---
 arch/alpha/kernel/syscalls/syscall.tbl      |    1 +
 arch/arm/tools/syscall.tbl                  |    1 +
 arch/arm64/tools/syscall_32.tbl             |    1 +
 arch/m68k/kernel/syscalls/syscall.tbl       |    1 +
 arch/microblaze/kernel/syscalls/syscall.tbl |    1 +
 arch/mips/kernel/syscalls/syscall_n32.tbl   |    1 +
 arch/mips/kernel/syscalls/syscall_n64.tbl   |    1 +
 arch/mips/kernel/syscalls/syscall_o32.tbl   |    1 +
 arch/parisc/kernel/syscalls/syscall.tbl     |    1 +
 arch/powerpc/kernel/syscalls/syscall.tbl    |    1 +
 arch/s390/kernel/syscalls/syscall.tbl       |    1 +
 arch/sh/kernel/syscalls/syscall.tbl         |    1 +
 arch/sparc/kernel/syscalls/syscall.tbl      |    1 +
 arch/x86/entry/syscalls/syscall_32.tbl      |    1 +
 arch/x86/entry/syscalls/syscall_64.tbl      |    1 +
 arch/xtensa/kernel/syscalls/syscall.tbl     |    1 +
 include/linux/syscalls.h                    |    1 +
 include/uapi/asm-generic/unistd.h           |    5 ++++-
 kernel/rseq.c                               |    9 +++++++++
 kernel/sys_ni.c                             |    1 +
 scripts/syscall.tbl                         |    1 +
 21 files changed, 32 insertions(+), 1 deletion(-)

--- a/arch/alpha/kernel/syscalls/syscall.tbl
+++ b/arch/alpha/kernel/syscalls/syscall.tbl
@@ -509,3 +509,4 @@
 577	common	open_tree_attr			sys_open_tree_attr
 578	common	file_getattr			sys_file_getattr
 579	common	file_setattr			sys_file_setattr
+580	common	rseq_slice_yield		sys_rseq_slice_yield
--- a/arch/arm/tools/syscall.tbl
+++ b/arch/arm/tools/syscall.tbl
@@ -484,3 +484,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	rseq_slice_yield		sys_rseq_slice_yield
--- a/arch/arm64/tools/syscall_32.tbl
+++ b/arch/arm64/tools/syscall_32.tbl
@@ -481,3 +481,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	rseq_slice_yield		sys_rseq_slice_yield
--- a/arch/m68k/kernel/syscalls/syscall.tbl
+++ b/arch/m68k/kernel/syscalls/syscall.tbl
@@ -469,3 +469,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	rseq_slice_yield		sys_rseq_slice_yield
--- a/arch/microblaze/kernel/syscalls/syscall.tbl
+++ b/arch/microblaze/kernel/syscalls/syscall.tbl
@@ -475,3 +475,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	rseq_slice_yield		sys_rseq_slice_yield
--- a/arch/mips/kernel/syscalls/syscall_n32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n32.tbl
@@ -408,3 +408,4 @@
 467	n32	open_tree_attr			sys_open_tree_attr
 468	n32	file_getattr			sys_file_getattr
 469	n32	file_setattr			sys_file_setattr
+470	common	rseq_slice_yield		sys_rseq_slice_yield
--- a/arch/mips/kernel/syscalls/syscall_n64.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n64.tbl
@@ -384,3 +384,4 @@
 467	n64	open_tree_attr			sys_open_tree_attr
 468	n64	file_getattr			sys_file_getattr
 469	n64	file_setattr			sys_file_setattr
+470	common	rseq_slice_yield		sys_rseq_slice_yield
--- a/arch/mips/kernel/syscalls/syscall_o32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_o32.tbl
@@ -457,3 +457,4 @@
 467	o32	open_tree_attr			sys_open_tree_attr
 468	o32	file_getattr			sys_file_getattr
 469	o32	file_setattr			sys_file_setattr
+470	common	rseq_slice_yield		sys_rseq_slice_yield
--- a/arch/parisc/kernel/syscalls/syscall.tbl
+++ b/arch/parisc/kernel/syscalls/syscall.tbl
@@ -468,3 +468,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	rseq_slice_yield		sys_rseq_slice_yield
--- a/arch/powerpc/kernel/syscalls/syscall.tbl
+++ b/arch/powerpc/kernel/syscalls/syscall.tbl
@@ -560,3 +560,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	nospu	rseq_slice_yield		sys_rseq_slice_yield
--- a/arch/s390/kernel/syscalls/syscall.tbl
+++ b/arch/s390/kernel/syscalls/syscall.tbl
@@ -472,3 +472,4 @@
 467  common	open_tree_attr		sys_open_tree_attr		sys_open_tree_attr
 468  common	file_getattr		sys_file_getattr		sys_file_getattr
 469  common	file_setattr		sys_file_setattr		sys_file_setattr
+470  common	rseq_slice_yield	sys_rseq_slice_yield		sys_rseq_slice_yield
--- a/arch/sh/kernel/syscalls/syscall.tbl
+++ b/arch/sh/kernel/syscalls/syscall.tbl
@@ -473,3 +473,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	rseq_slice_yield		sys_rseq_slice_yield
--- a/arch/sparc/kernel/syscalls/syscall.tbl
+++ b/arch/sparc/kernel/syscalls/syscall.tbl
@@ -515,3 +515,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	rseq_slice_yield		sys_rseq_slice_yield
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -475,3 +475,4 @@
 467	i386	open_tree_attr		sys_open_tree_attr
 468	i386	file_getattr		sys_file_getattr
 469	i386	file_setattr		sys_file_setattr
+470	i386	rseq_slice_yield	sys_rseq_slice_yield
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -393,6 +393,7 @@
 467	common	open_tree_attr		sys_open_tree_attr
 468	common	file_getattr		sys_file_getattr
 469	common	file_setattr		sys_file_setattr
+470	common	rseq_slice_yield	sys_rseq_slice_yield
 
 #
 # Due to a historical design error, certain syscalls are numbered differently
--- a/arch/xtensa/kernel/syscalls/syscall.tbl
+++ b/arch/xtensa/kernel/syscalls/syscall.tbl
@@ -440,3 +440,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	rseq_slice_yield		sys_rseq_slice_yield
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -957,6 +957,7 @@ asmlinkage long sys_statx(int dfd, const
 			  unsigned mask, struct statx __user *buffer);
 asmlinkage long sys_rseq(struct rseq __user *rseq, uint32_t rseq_len,
 			 int flags, uint32_t sig);
+asmlinkage long sys_rseq_slice_yield(void);
 asmlinkage long sys_open_tree(int dfd, const char __user *path, unsigned flags);
 asmlinkage long sys_open_tree_attr(int dfd, const char __user *path,
 				   unsigned flags,
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -858,8 +858,11 @@
 #define __NR_file_setattr 469
 __SYSCALL(__NR_file_setattr, sys_file_setattr)
 
+#define __NR_rseq_slice_yield 470
+__SYSCALL(__NR_rseq_slice_yield, sys_rseq_slice_yield)
+
 #undef __NR_syscalls
-#define __NR_syscalls 470
+#define __NR_syscalls 471
 
 /*
  * 32 bit systems traditionally used different
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -542,6 +542,15 @@ int rseq_slice_extension_prctl(unsigned
 	return -EFAULT;
 }
 
+SYSCALL_DEFINE0(rseq_slice_yield)
+{
+	if (need_resched()) {
+		schedule();
+		return 1;
+	}
+	return 0;
+}
+
 static int __init rseq_slice_cmdline(char *str)
 {
 	bool on;
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -390,5 +390,6 @@ COND_SYSCALL(setuid16);
 
 /* restartable sequence */
 COND_SYSCALL(rseq);
+COND_SYSCALL(rseq_sched_yield);
 
 COND_SYSCALL(uretprobe);
--- a/scripts/syscall.tbl
+++ b/scripts/syscall.tbl
@@ -410,3 +410,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	rseq_sched_yield		sys_rseq_sched_yield

Hello Thomas,

On 9/9/2025 4:30 AM, Thomas Gleixner wrote:
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -542,6 +542,15 @@ int rseq_slice_extension_prctl(unsigned
>  	return -EFAULT;
>  }
>  

nit.

Perhaps a small note here to highlight how need_resched() is true
for tasks who had the slice extension granted. Something like:

/**
 * sys_rseq_slice_yield - yield the current processor if a task granted with
 * slice extension is done with the critical work before being forced out.
 *
 * This syscall entry work ensures NEED_RESCHED is set if the task was granted
 * a slice extension before arriving here.
 *
 * Return: 1 if the task successfully yielded the CPU within the granted slice.
 *	   0 if the slice extension was either never granted or was revoked by
 *	   going over the granted extension.
 */

> +SYSCALL_DEFINE0(rseq_slice_yield)
> +{
> +	if (need_resched()) {
> +		schedule();
> +		return 1;
> +	}
> +	return 0;
> +}

-- 
Thanks and Regards,
Prateek

Hello Thomas,

On 9/9/2025 4:30 AM, Thomas Gleixner wrote:
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -542,6 +542,15 @@ int rseq_slice_extension_prctl(unsigned
>  	return -EFAULT;
>  }
>  
> +SYSCALL_DEFINE0(rseq_slice_yield)
> +{
> +	if (need_resched()) {
> +		schedule();
> +		return 1;
> +	}
> +	return 0;
> +}
> +
>  static int __init rseq_slice_cmdline(char *str)
>  {
>  	bool on;
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -390,5 +390,6 @@ COND_SYSCALL(setuid16);
>  
>  /* restartable sequence */
>  COND_SYSCALL(rseq);
> +COND_SYSCALL(rseq_sched_yield);

I'm not sure if it is my toolchain but when I try to build a version
with CONFIG_RSEQ_SLICE_EXTENSION disabled, I see:

    ld: vmlinux.o: in function `x64_sys_call':
    arch/x86/include/generated/asm/syscalls_64.h:471: undefined reference to `__x64_sys_rseq_slice_yield'
    ld: vmlinux.o: in function `ia32_sys_call':
    arch/x86/include/generated/asm/syscalls_32.h:471: undefined reference to `__ia32_sys_rseq_slice_yield'
    ld: vmlinux.o:(.rodata+0x12d0): undefined reference to `__x64_sys_rseq_slice_yield'

I would have assumed the COND_SYSCALL() above would have stubbed this
but that doesn't seem to be the case. Am I missing something?
P.S. I'm running with:

    gcc version 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04.2)
    GNU ld (GNU Binutils for Ubuntu) 2.38

-- 
Thanks and Regards,
Prateek

On Tue, Sep 09 2025 at 15:22, K. Prateek Nayak wrote:
> On 9/9/2025 4:30 AM, Thomas Gleixner wrote:
>>  /* restartable sequence */
>>  COND_SYSCALL(rseq);
>> +COND_SYSCALL(rseq_sched_yield);
>
> I'm not sure if it is my toolchain but when I try to build a version
> with CONFIG_RSEQ_SLICE_EXTENSION disabled, I see:
>
>     ld: vmlinux.o: in function `x64_sys_call':
>     arch/x86/include/generated/asm/syscalls_64.h:471: undefined reference to `__x64_sys_rseq_slice_yield'
>     ld: vmlinux.o: in function `ia32_sys_call':
>     arch/x86/include/generated/asm/syscalls_32.h:471: undefined reference to `__ia32_sys_rseq_slice_yield'
>     ld: vmlinux.o:(.rodata+0x12d0): undefined reference to `__x64_sys_rseq_slice_yield'
>
> I would have assumed the COND_SYSCALL() above would have stubbed this
> but that doesn't seem to be the case. Am I missing something?

Yes.

>> +COND_SYSCALL(rseq_sched_yield);

does not create a stub for rseq_slice_yield() obviously :)

/me looks for a brown paperbag.