Re: [PATCH RFC cpumask 4/5] cpumask: Add "last" alias for cpu list specifications

[Peter Zijlstra]

On Tue, Jan 05, 2021 at 04:49:55PM -0800, paulmck@xxxxxxxxxx wrote:
> From: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
> 
> 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.