  The computing resource needs of LHC experiments are expected to continue
growing significantly during the Run 3 and into the HL-LHC era. The landscape
of available resources will also evolve, as High Performance Computing (HPC)
and Cloud resources will provide a comparable, or even dominant, fraction of
the total compute capacity. The future years present a challenge for the
experiments' resource provisioning models, both in terms of scalability and
increasing complexity. The CMS Submission Infrastructure (SI) provisions
computing resources for CMS workflows. This infrastructure is built on a set of
federated HTCondor pools, currently aggregating 400k CPU cores distributed
worldwide and supporting the simultaneous execution of over 200k computing
tasks. Incorporating HPC resources into CMS computing represents firstly an
integration challenge, as HPC centers are much more diverse compared to Grid
sites. Secondly, evolving the present SI, dimensioned to harness the current
CMS computing capacity, to reach the resource scales required for the HLLHC
phase, while maintaining global flexibility and efficiency, will represent an
additional challenge for the SI. To preventively address future potential
scalability limits, the SI team regularly runs tests to explore the maximum
reach of our infrastructure. In this note, the integration of HPC resources
into CMS offline computing is summarized, the potential concerns for the SI
derived from the increased scale of operations are described, and the most
recent results of scalability test on the CMS SI are reported.
