On Tue, Jun 30, 2020 at 04:25:23PM +0100, Ankur Sinha wrote:
> On Mon, Jun 29, 2020 15:01:24 -0600, Chris Murphy wrote:
> > https://bugzilla.redhat.com/show_bug.cgi?id=1851783
> > 
> > The main argument is that for typical and varied workloads in Fedora,
> > mostly on consumer hardware, we should use mq-deadline scheduler
> > rather than either none or bfq.
> > 
> > It may be true most folks with NVMe won't see anything bad with none,
> > but those who have heavier IO workloads are likely to be better off
> > with mq-deadline.
> > 
> > Further details are in the bug, but let's discuss it on list. Thanks!
> There was this thread about our systems hanging, and the workaround was
> to revert to mq-deadline from bfq:
> https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/thread/MJJFT5AOYUFZ3SO2EDVLJSDAZMZI4HAP/#DA7RCQFIAD4Z3Q7HQBW2ELPTLPYDKJMT

To clarify: you could reliably reproduce the issue when building steps in mock.
Did you verify that it is reliably fixed simply by switching bfq→mq-deadline?


> There are a few threads on AskFedora about systems hanging. They're not
> the easiest to debug but we did suggest people try switching to
> mq-deadline to see if it helps:
> https://ask.fedoraproject.org/t/whole-os-freezes-watching-a-video-with-mpv/6770/10
> I don't know enough about this to say if it's a bug and if it has been
> fixed.

There's a lot of noise in those bug reports. For heisenbugs, the fact
that something was an issue and after a flurry of half-random changes
to the system isn't, does not allow us conclude _anything_. We need
somebody who understands what they are doing to isolate the issue. In
particular, if this is a kernel hang, than we need a proper traceback
from the kernel, and not just assume it's the scheduler.
(In particular, if this is a race condition, changing the scheduler
could be just making the condition less likely because the system is
slower or faster or just schedules processes in a different order,
without the scheduler being relevant to the bug).

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