Re: [patch V2 08/12] rseq: Implement time slice extension enforcement timer

[K Prateek Nayak]

On 10/28/2025 2:30 PM, Sebastian Andrzej Siewior wrote:
>>> Without an interrupt on the target CPU, there is nothing stopping the
>>> task from overstepping its fair share.
>>
>> When the task moves CPU, the rseq_exit_user_update() would clear all
>> of the slice extension state before running the task again. The task
>> will start off again with "rseq->slice_ctrl.request" and
>> "rseq->slice_ctrl.granted" both at 0 signifying the task was
>> rescheduled.
> 
> I wasn't aware this is done once the task is in userland and then
> relocated to another CPU.

The exact path based on my understanding is:

  /* Task migrates to another CPU; Has to resume from kernel. */
  __schedule()
    context_switch()
      rseq_sched_switch_event()
        t->rseq.event.sched_switch = true;
        set_tsk_thread_flag(t, TIF_RSEQ);

    ...
    exit_to_user_mode_loop()
      rseq_exit_to_user_mode_restart()
        __rseq_exit_to_user_mode_restart()
          /* Sees t->rseq.event.sched_switch to be true. */
          rseq_exit_user_update()
            if (rseq_slice_extension_enabled())
              unsafe_put_user(0U, &rseq->slice_ctrl.all, efault); /* Unconditionally clears all of "rseq_ctrl" */

-- 
Thanks and Regards,
Prateek