I'm resending this patch proposal after having addressed a few Juri's comments, and a rebase on top of the post-august-break tip sched/core. The SCHED_DEADLINE scheduler allows reading the statically configured run-time, deadline, and period parameters through the sched_getattr() system call. However, there is no immediate way to access, from user space, the current parameters used within the scheduler: the instantaneous runtime left in the current cycle, as well as the current absolute deadline. The `flags' sched_getattr() parameter, so far mandated to contain zero, now supports the SCHED_GETATTR_FLAG_DL_DYNAMIC=1 flag, to request retrieval of the leftover runtime and absolute deadline, converted to a CLOCK_MONOTONIC reference, instead of the statically configured parameters. This feature is useful for adaptive SCHED_DEADLINE tasks that need to modify their behavior depending on whether or not there is enough runtime left in the current period, and/or what is the current absolute deadline. Notes: - before returning the instantaneous parameters, the runtime is updated; - the abs deadline is returned shifted from rq_clock() to ktime_get_ns(), in CLOCK_MONOTONIC reference; this causes multiple invocations from the same period to return values that may differ for a few ns (showing some small drift), albeit the deadline doesn't move, in rq_clock() reference; - the abs deadline value returned to user-space, as unsigned 64-bit value, can represent nearly 585 years since boot time; - setting flags=0 provides the old behavior (retrieve static parameters). See also the notes from discussion held at OSPM 2025 on the topic "Making user space aware of current deadline-scheduler parameters". Signed-off-by: Tommaso Cucinotta <tommaso.cucinotta@xxxxxxxxxxxxxxx>
