Re: [PATCH v3 5/5] lib/cpumask: reorganize cpumask_local_spread() logic

Peter Lafreniere
2022-12-08T20:58:07Z

> On Thu, Dec 08, 2022 at 08:17:22PM +0000, Peter Lafreniere wrote:
> > > Now after moving all NUMA logic into sched_numa_find_nth_cpu(),
> > > else-branch of cpumask_local_spread() is just a function call, and
> > > we can simplify logic by using ternary operator.
> > >
> > > While here, replace BUG() with WARN().
> > Why make this change? It's still as bad to hit the WARN_ON as it was before.
>
> For example, because of this:
>
> > Greg, please don't do this
> >
> > > ChangeSet@1.614, 2002-09-05 08:33:20-07:00, greg@kroah.com
> > > USB: storage driver: replace show_trace() with BUG()
> >
> > that BUG() thing is _way_ out of line, and has killed a few of my machines
> > several times for no good reason. It actively hurts debuggability, because
> > the machine is totally dead after it, and the whole and ONLY point of
> > BUG() messages is to help debugging and make it clear that we can't handle
> > something.
> >
> > In this case, we _can_ handle it, and we're much better off with a machine
> > that works and that you can look up the messages with than killing it.
> >
> > Rule of thumb: BUG() is only good for something that never happens and
> > that we really have no other option for (ie state is so corrupt that
> > continuing is deadly).
> >
> > Linus

Fair enough. It's not like it'll be hit anyway. My concern was for if
any of the 23 callers get an invalid result. I guess that if that causes
a crash, then so be it. We have the warning to track down the cause.

Thanks for the explanation,
Peter Lafreniere <peter@n8pjl.ca>