[$] Pressure stall monitors

One of the useful features added during the 4.20 development cycle was theavailability of pressure-stall information,which provides visibility into how resource-constrained the system is.Interest in using this information has spread beyond the data-centerenvironment where it was first implemented, but it turns out that there someshortcomings in the current interface that affect other use cases. SurenBaghdasaryan has posted a patchset aimed at making pressure-stall information more useful for theAndroid use case - and, most likely, for many other use cases as well.