On Tue, Jan 05, 2021 at 04:49:55PM -0800, paul...@kernel.org wrote: > From: Paul Gortmaker <paul.gortma...@windriver.com> > > It seems that a common configuration is to use the 1st couple cores > for housekeeping tasks, and or driving a busy peripheral that generates > a lot of interrupts, or something similar. > > This tends to leave the remaining ones to form a pool of similarly > configured cores to take on the real workload of interest to the user. > > So on machine A - with 32 cores, it could be 0-3 for "system" and then > 4-31 being used in boot args like nohz_full=, or rcu_nocbs= as part of > setting up the worker pool of CPUs. > > But then newer machine B is added, and it has 48 cores, and so while > the 0-3 part remains unchanged, the pool setup cpu list becomes 4-47. > > Deployment would be easier if we could just simply replace 31 and 47 > with "last" and let the system substitute in the actual number at boot; > a number that it knows better than we do. > > No need to have custom boot args per node, no need to do a trial boot > in order to snoop /proc/cpuinfo and/or /sys/devices/system/cpu - no > more fencepost errors of using 32 and 48 instead of 31 and 47. > > A generic token replacement is used to substitute "last" with the > number of CPUs present before handing off to bitmap processing. But > it could just as easily be used to replace any placeholder token with > any other token or value only known at/after boot.
Aside from the comments Yury made, on how all this is better in bitmap_parselist(), how about doing s/last/N/ here? For me something like: "4-N" reads much saner than "4-last". Also, it might make sense to teach all this about core/node topology, but that's going to be messy. Imagine something like "Core1-CoreN" or "Nore1-NodeN" to mean the mask all/{Core,Node}0. And that is another feature that seems to be missing from parselist, all/except.