[$] Pressure stall monitors

One of the useful features added during the 4.20 development cycle was the
availability of pressure-stall information,
which provides visibility into how resource-constrained the system is.
Interest in using this information has spread beyond the data-center
environment where it was first implemented, but it turns out that there some
shortcomings in the current interface that affect other use cases. Suren
Baghdasaryan has posted a patch
set aimed at making pressure-stall information more useful for the
Android use case — and, most likely, for many other use cases as well.