Cessna 172 pilot reported experiencing a rough running engine; resulting in an immediate return to the departure airport; where a successful landing was accomplished.

Narrative

I did the normal preflight inspection; taxied out and departed ZZZ on Runway XX. After leveling out at 3500 MSL (roughly over the quarry) and reducing power for level flight; the engine suddenly began to run rough; as though one or two cylinders were not firing. The engine was still making power; just diminished and running rough.I verified that the fuel tank selector was on 'Both'.Oil pressure and oil temperature were normal.I pulled carburetor heat with the usual RPM drop but no change in the engine roughness. I cycled the mags from both to left and right and back to both; with no change other than the usual RPM drop. I leaned the mixture until RPM dropped and returned to full rich; again with no change.At that point I turned back toward ZZZ; changed frequency to 121.50 MHz and requested priority handling.An airliner; acknowledged and relayed into to ZZZ Center.From 3500 MSL and only 8 miles from ZZZ; I had no trouble returning to the airport; entered a normal downwind pattern for Runway XX and landed without incident. I do not recall if I switched frequencies back to the CTAF to announce arrival when I entered the downwind. I called ZZZ Center on 121.50 during rollout and advised that I had landed at ZZZ.The engine continued to run rough all the way to shutdown on the apron.There was no other traffic until 10 minutes or so after I had landed; a Cessna 206 Skywagon came in for fuel as I was pushing the aircraft back into the T-hangar.There was no oil leakage inside the cowling or underneath the aircraft. Oil level was still at just under 7 quarts.I called the aircraft's owner to advise him of the situation and that his plane was back on the ground safely.ZZZ Center called and I verified that the situation had concluded without incident.