Re: [PATCH] powerpc/smp: poll cpu_callin_map more aggressively in __cpu_up()

Michael Ellerman [off-list ref] writes:

Nathan Lynch [off-list ref] writes:

quoted

Replace the outdated iteration and timeout calculations here with
indefinite spin_until_cond()-wrapped poll of cpu_callin_map. __cpu_up()
already does this when waiting for the cpu to set its online bit before
returning, so this change is not really making the function more brittle.

I'm not sure I agree that this doesn't make the code more brittle.

The existing indefinite wait you mention is later in the function, and
happens after the CPU has successfully come into the kernel.

I think it's more common that a stuck/borked CPU doesn't come into the
kernel at all, rather than comes in and then fails to online.

So I think the bail out when the CPU fails to call in is useful, I would
guess I see that "Processor x is stuck" message multiple times a year
while debugging various things.

Yeah I can see how my claim is too strong here.

quoted

Removing the msleep(1) in the hotplug path here reduces the time it takes
to online a CPU on a P9 PowerVM LPAR from about 30ms to 1ms when exercised
via thaw_secondary_cpus().

That is a nice improvement.

Can we do something that returns quickly in the happy case and still has
a timeout when things go wrong? Seems like a busy loop with a
time_after() check would do the trick.

Yes, I'll rework it like that. Thanks.