Avoid UE iterations in the scheduler

[sgarg00]

Originally reported by Robert Schmidt on 2026-05-15 — GitLab #1094

Converted from comment https://gitlab.eurecom.fr/oai/openairinterface5g/-/merge_requests/3575#note_219359

In various places in the scheduler (e.g., nr_csirs_scheduling(), nr_csi_meas_reporting(), nr_schedule_srs(), etc.), we iterate over all UEs in each slot to verify that a given condition for individual UEs is valid. For many UEs (say, 100), that might be costly. In the three mentioned cases above, it comes down to something like

(frame * n_slots_frame + slot - offset) % period == 0

i.e., "is the current frame/slot the one in which this UE is configured to send/receive X"?

That is inefficient. In these cases, we have a clear frame/slot <-> UE association (possibly multiple UEs per frame/slot).

Instead, I suggest that for kind of periodic scheduling, we should instead maintain an array indexed by time (frame/slot) mapping to N UEs that need to be scheduled. For instance, in SR scheduleing [from the comment mentioned above], maintain

NR_UE_info_t *sr_per_UE[MAX_SLOTS_SR_PERIDIOCITY];

then lookup at given frame/slot

int slot = convert_to_slot_max_sr_periodicity(frame, slot);
NR_UE_info_t *srUE = sr_per_UE[slot];
// srUE != NULL => UE has SR in this slot

to lookup if a given UE is to be scheduled SR, then allocate. That translates the O(n) operation into O(1).