ftrace hangs waiting for rcu

[Sven Schnelle]

Hi Mark,

Mark Rutland <mark.rutland at arm.com> writes:

> On Fri, Jan 28, 2022 at 05:08:48PM +0100, Sven Schnelle wrote:
>> We noticed the PR from Paul and are currently testing the fix. So far
>> it's looking good. The configuration where we have seen the hang is a
>> bit unusual:
>> 
>> - 16 physical CPUs on the kvm host
>> - 248 logical CPUs inside kvm
>
> Aha! 248 is notably *NOT* a power of two, and in this case the shift would be
> wrong (ilog2() would give 7, when we need a shift of 8).
>
> So I suspect you're hitting the same issue as I was.

Argh, indeed! I somehow changed 'power of two' to 'odd number' in my
head. I guess it's time for the weekend. :-)

Thanks!