Re: [PATCH v2 02/23] cpu/hotplug: Avoid trying to bring up CPUs that are already online

[Thomas Gleixner]

On Fri, Oct 09 2026 at 11:07, Will Deacon wrote:
> There's little point trying to bring up a CPU that is already online.
> Although _cpu_up() handles this case by doing nothing (because the
> target state has already been reached), it's wasted effort when we can
> easily elide the call to cpu_up() in the first place.

s/we can ../the call can be .../

> Check that the target CPU isn't already online before invoking cpu_up()
> from cpuhp_bringup_mask().

Confused.

cpuhp_bringup_mask() is only invoked from bringup_nonboot_cpus() during
init. So there is only _ONE_ CPU which can be online and in the present
mask, i.e. the boot CPU, right? Any other CPU than that being online
would be a bug, no?

Can you spell that out please and add a comment to that effect?

I'm not opposed to the change per se, but I doubt that the "win" is
actually measurable as you trade that one pointless call against a
conditional which has to be executed NR_CPUS times, where NR_CPUS-1
times the conditinal is pointless. There is ZERO contention at that
point, so what's the actual benefit?

Thanks,

        tglx