DL inference queries play an important role in diverse internet services and a large fraction of datacenter cycles are spent on processing DL inference queries. Specifically, the matrix-matrix multiplication (GEMM) operations of fully-connected MLP layers dominate many inference tasks. We find that the GEMM operations for datacenter DL inference tasks are memory bandwidth bound, contrary to common assumptions: (1) strict query latency constraints force small-batch operation, which limits reuse and increases bandwidth demands; and (2) large and colocated models require reading the large weight matrices from main memory, again requiring high bandwidth without offering reuse opportunities. We demonstrate the large potential of accelerating these small-batch GEMMs with processing in the main CPU memory. We develop a novel GEMM execution flow and corresponding memory-side address-generation logic that exploits GEMM locality and enables long-running PIM kernels despite the complex address-mapping functions employed by the CPU that would otherwise destroy locality. Our evaluation of StepStone variants at the channel, device, and within-device PIM levels, along with optimizations that balance parallelism benefits with data-distribution overheads demonstrate $12\times$ better minimum latency than a CPU and $2.8\times$ greater throughput for strict query latency constraints. End-to-end performance analysis of recent recommendation and language models shows that StepStone PIM outperforms a fast CPU (by up to $16\times$) and prior main-memory acceleration approaches (by up to $2.4\times$ compared to the best prior approach).