Re: [PATCH] powerpc/pseries/hotplug-cpu: increase wait time for vCPU death

Michael Roth [off-list ref] writes:

Quoting Michael Roth (2020-08-04 23:37:32)

quoted

Quoting Michael Ellerman (2020-08-04 22:07:08)

quoted

Greg Kurz [off-list ref] writes:

quoted

On Tue, 04 Aug 2020 23:35:10 +1000
Michael Ellerman [off-list ref] wrote:

quoted

Spinning forever seems like a bad idea, but as has been demonstrated at
least twice now, continuing when we don't know the state of the other
CPU can lead to straight up crashes.

So I think I'm persuaded that it's preferable to have the kernel stuck
spinning rather than oopsing.

+1

quoted

I'm 50/50 on whether we should have a cond_resched() in the loop. My
first instinct is no, if we're stuck here for 20s a stack trace would be
good. But then we will probably hit that on some big and/or heavily
loaded machine.

So possibly we should call cond_resched() but have some custom logic in
the loop to print a warning if we are stuck for more than some
sufficiently long amount of time.

How long should that be ?

Yeah good question.

I guess step one would be seeing how long it can take on the 384 vcpu
machine. And we can probably test on some other big machines.

Hopefully Nathan can give us some idea of how long he's seen it take on
large systems? I know he was concerned about the 20s timeout of the
softlockup detector.

Maybe a minute or two?

Hmm, so I took a stab at this where I called cond_resched() after
every 5 seconds of polling and printed a warning at the same time (FWIW
that doesn't seem to trigger any warnings on a loaded 96-core mihawk
system using KVM running the 384vcpu unplug loop)

But it sounds like that's not quite what you had in mind. How frequently
do you think we should call cond_resched()? Maybe after 25 iterations
of polling smp_query_cpu_stopped() to keep original behavior somewhat
similar?

I think we can just call it on every iteration, it should be cheap
compared to an RTAS call.

The concern was just by doing that you effectively prevent the
softlockup detector from reporting you as stuck in that path. Hence the
desire to manually print a warning after ~60s or something.

cheers