Re: Endless "reclaiming chunk"/"relocating block group"

[Zygo Blaxell <ce3g8jdj@umail.furryterror.org>]

On Thu, Oct 20, 2022 at 01:01:48PM +0200, Christoph Biedl wrote:
> Thanks for add the toughts shared ...
> 
> Zygo Blaxell wrote...
> 
> (...)
> > It makes me think of possible rounding errors (e.g. the threshold
> > calculation divides by 100, or there's a sum of quantities that leads
> > to a percentage > 100, but the code treats zero as a special case and
> > bails out long before, so I don't see how we'd reach those corner cases.
> 
> Out of desperation, I followed an approach I could read between the lines
> here. The thing I did not mention (just because I forgot about it) is
> the compiler version used to build the kernel. For reasons, this is
> still a gcc-5 (5.4.0, to be precise). After a rebuild using gcc-11,
> things are nice and smooth.

That's...not the weirdest thing I've ever seen, but maybe the weirdest
thing I've seen this month.

> Now I cannot deny I'm quite confused. Assuming this is really the cause,
> how should I go from here? I can patch the sources at will, but it's a
> huge amount of code, and I don't even know where is start. Also from
> expericence, debug print statements do bar the optimizer from creating
> the problematic instructions.

That in itself would (weakly) confirm a compiler issue.  But it might
be simpler than that, like a bug in the implementation of a math builtin
that wasn't popular in the kernel 8 years ago, but is popular now after
the implementation was debugged in GCC.  I don't know if that's the case
in this instance, but similar things have happened in the past.

>     Christoph
> 
> FWIW, according to Documentation/process/changes.rst, that old gcc-5 is
> considered sufficient. I didn't expect that :)

On paper maybe, but maybe not in real-world use.  Somewhat related thread
on linux-kernel:

	https://lore.kernel.org/all/Y0hz3u8ZNO2yFU2f@sirena.org.uk/T/

TL;DR not enough people are testing new kernel code against old compilers.
If it's problematic to keep the host system's gcc up to date, a build
chroot for kernel building with an up to date toolchain is the way to go.