Diddy and Mase Reunion -- 'Mo Money, Less Problems ... Bad Ass Performance

The 90's were on and popping again Friday night in L.A. during Dr. Dre's pre-Grammy Party ... but nothing was cooler than Puff Daddy reuniting on stage with his former protege Ma$e.

Diddy and Ma$e killed a stellar set of a songs including "Been Around the World" and "All About the Benjamins" at the Belasco Theatre ... but their rendition of "'Mo Money, 'Mo Problems" was off the charts.

Eminem hit the stage later in the evening with Dr. Dre ... who also reunited with Blackstreet to perform "No Diggity."

It was all that and a bag of chips.