Re: [PATCH] timer: fix NR_CPUS=1 build with gcc13

[George Dunlap <george.dunlap@xxxxxxxxx>]

On Wed, Sep 13, 2023 at 8:32 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> Gcc13 apparently infers from "if ( old_cpu < new_cpu )" that "new_cpu"
> is >= 1, and then (on x86) complains about "per_cpu(timers, new_cpu)"
> exceeding __per_cpu_offset[]'s bounds (being an array of 1 in such a
> configuration). Make the code conditional upon there being at least 2
> CPUs configured (otherwise there simply is nothing to migrate [to]).

Hmm, without digging into it, migrate_timer() doesn't seem like very
robust code: It doesn't check to make sure that new_cpu is valid, nor
does it give the option of returning an error if anything fails.
Making migrate_timer() just do nothing on 1-cpu systems seems will
remove the warning, but not really make things safer.

Is this a super hot path?  Would it make more sense to add `||
(new_cpu > CONFIG_NR_CPUS)` to the early-return  conditional at the
top of the first `for (; ; )` loop?

I guess if we don't expect it ever to be called, it might be better to
get rid of the code entirely; but maybe in that case we should add
something like the following?

```
#else
    WARN_ONCE("migrate_timer: Request to move to %u on a single-core
system!", new_cpu);
    ASSERT_UNREACHABLE();
#endif
```

Thoughts?

 -George