rseq: Implement time slice extension enforcement timer

rseq: Implement time slice extension enforcement timer

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 <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251215155709.068329497@linutronix.de