linux-toradex.git/kernel/rseq.c, branch v7.0-rc1

rseq: Lower default slice extension

2026-01-22T10:11:20+00:00

Change the minimum slice extension to 5 usec.

Since slice_test selftest reaches a staggering ~350 nsec extension:

Task: slice_test    Mean: 350.266 ns
  Latency (us)    | Count
  ------------------------------
  EXPIRED         | 238
  0 us            | 143189
  1 us            | 167
  2 us            | 26
  3 us            | 11
  4 us            | 28
  5 us            | 31
  6 us            | 22
  7 us            | 23
  8 us            | 32
  9 us            | 16
  10 us           | 35

Lower the minimal (and default) value to 5 usecs -- which is still massive.

Signed-off-by: Peter Zijlstra (Intel) 
Link: https://patch.msgid.link/20260121143208.073200729@infradead.org

rseq: Move slice_ext_nsec to debugfs

2026-01-22T10:11:20+00:00

Move changing the slice ext duration to debugfs, a sliglty less permanent
interface.

Signed-off-by: Peter Zijlstra (Intel) 
Link: https://patch.msgid.link/20260121143207.923520192@infradead.org

rseq: Allow registering RSEQ with slice extension

2026-01-22T10:11:19+00:00

Since glibc cares about the number of syscalls required to initialize a new
thread, allow initializing rseq with slice extension on. This avoids having to
do another prctl().

Requested-by: Mathieu Desnoyers 
Signed-off-by: Peter Zijlstra (Intel) 
Link: https://patch.msgid.link/20260121143207.814193010@infradead.org

rseq: Implement time slice extension enforcement timer

2026-01-22T10:11:18+00:00

If a time slice extension is granted and the reschedule delayed, the kernel
has to ensure that user space cannot abuse the extension and exceed the
maximum granted time.

It was suggested to implement this via the existing hrtick() timer in the
scheduler, but that turned out to be problematic for several reasons:

   1) It creates a dependency on CONFIG_SCHED_HRTICK, which can be disabled
      independently of CONFIG_HIGHRES_TIMERS

   2) HRTICK usage in the scheduler can be runtime disabled or is only used
      for certain aspects of scheduling.

   3) The function is calling into the scheduler code and that might have
      unexpected consequences when this is invoked due to a time slice
      enforcement expiry. Especially when the task managed to clear the
      grant via sched_yield(0).

It would be possible to address #2 and #3 by storing state in the
scheduler, but that is extra complexity and fragility for no value.

Implement a dedicated per CPU hrtimer instead, which is solely used for the
purpose of time slice enforcement.

The timer is armed when an extension was granted right before actually
returning to user mode in rseq_exit_to_user_mode_restart().

It is disarmed, when the task relinquishes the CPU. This is expensive as
the timer is probably the first expiring timer on the CPU, which means it
has to reprogram the hardware. But that's less expensive than going through
a full hrtimer interrupt cycle for nothing.

Signed-off-by: Thomas Gleixner 
Signed-off-by: Peter Zijlstra (Intel) 
Reviewed-by: Mathieu Desnoyers 
Link: https://patch.msgid.link/20251215155709.068329497@linutronix.de

rseq: Implement syscall entry work for time slice extensions

2026-01-22T10:11:18+00:00

The kernel sets SYSCALL_WORK_RSEQ_SLICE when it grants a time slice
extension. This allows to handle the rseq_slice_yield() syscall, which is
used by user space to relinquish the CPU after finishing the critical
section for which it requested an extension.

In case the kernel state is still GRANTED, the kernel resets both kernel
and user space state with a set of sanity checks. If the kernel state is
already cleared, then this raced against the timer or some other interrupt
and just clears the work bit.

Doing it in syscall entry work allows to catch misbehaving user space,
which issues an arbitrary syscall, i.e. not rseq_slice_yield(), from the
critical section. Contrary to the initial strict requirement to use
rseq_slice_yield() arbitrary syscalls are not considered a violation of the
ABI contract anymore to allow onion architecture applications, which cannot
control the code inside a critical section, to utilize this as well.

If the code detects inconsistent user space that result in a SIGSEGV for
the application.

If the grant was still active and the task was not preempted yet, the work
code reschedules immediately before continuing through the syscall.

Signed-off-by: Thomas Gleixner 
Signed-off-by: Peter Zijlstra (Intel) 
Link: https://patch.msgid.link/20251215155709.005777059@linutronix.de

rseq: Implement sys_rseq_slice_yield()

2026-01-22T10:11:17+00:00

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 
Signed-off-by: Peter Zijlstra (Intel) 
Reviewed-by: Mathieu Desnoyers 
Acked-by: Arnd Bergmann 
Link: https://patch.msgid.link/20251215155708.929634896@linutronix.de

rseq: Add prctl() to enable time slice extensions

2026-01-22T10:11:17+00:00

Implement a prctl() so that tasks can enable the time slice extension
mechanism. This fails, when time slice extensions are disabled at compile
time or on the kernel command line and when no rseq pointer is registered
in the kernel.

That allows to implement a single trivial check in the exit to user mode
hotpath, to decide whether the whole mechanism needs to be invoked.

Signed-off-by: Thomas Gleixner 
Signed-off-by: Peter Zijlstra (Intel) 
Link: https://patch.msgid.link/20251215155708.858717691@linutronix.de

rseq: Add statistics for time slice extensions

2026-01-22T10:11:17+00:00

Extend the quick statistics with time slice specific fields.

Signed-off-by: Thomas Gleixner 
Signed-off-by: Peter Zijlstra (Intel) 
Link: https://patch.msgid.link/20251215155708.795202254@linutronix.de

rseq: Provide static branch for time slice extensions

2026-01-22T10:11:16+00:00

Guard the time slice extension functionality with a static key, which can
be disabled on the kernel command line.

Signed-off-by: Thomas Gleixner 
Signed-off-by: Peter Zijlstra (Intel) 
Link: https://patch.msgid.link/20251215155708.733429292@linutronix.de

rseq: Add fields and constants for time slice extension

2026-01-22T10:11:16+00:00

Aside of a Kconfig knob add the following items:

   - Two flag bits for the rseq user space ABI, which allow user space to
     query the availability and enablement without a syscall.

   - A new member to the user space ABI struct rseq, which is going to be
     used to communicate request and grant between kernel and user space.

   - A rseq state struct to hold the kernel state of this

   - Documentation of the new mechanism

Signed-off-by: Thomas Gleixner 
Signed-off-by: Peter Zijlstra (Intel) 
Link: https://patch.msgid.link/20251215155708.669472597@linutronix.de