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

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Date:   Wed, 6 Jan 2021 13:16:50 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     paulmck@...nel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kernel-team@...com, Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: Re: [PATCH RFC cpumask 4/5] cpumask: Add "last" alias for cpu list specifications

On Wed, Jan 6, 2021 at 1:50 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Jan 05, 2021 at 04:49:55PM -0800, paulmck@...nel.org wrote:
> > From: Paul Gortmaker <paul.gortmaker@...driver.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.

If you just want to teach bitmap_parselist() to "s/Core0/0-4",  I think
it's doable if we add a hook to a proper subsystem in bitmap_parselist().

> And that is another feature that seems to be missing from parselist,
> all/except.

We already support groups in a range. I think it partially covers the
proposed all/except.

Can you share examples on what you miss?