On Fri, Mar 08, 2019 at 12:22:20PM -0500, Josef Bacik wrote:
> On Thu, Mar 07, 2019 at 07:08:31PM +0100, Andrea Righi wrote:
> > = Problem =
> >
> > When sync() is executed from a high-priority cgroup, the process is forced
> > to
> > wait the completion of the entire outstanding writeback I/O, even the I/O
> > that
> > was originally generated by low-priority cgroups potentially.
> >
> > This may cause massive latencies to random processes (even those running in
> > the
> > root cgroup) that shouldn't be I/O-throttled at all, similarly to a classic
> > priority inversion problem.
> >
> > This topic has been previously discussed here:
> > https://patchwork.kernel.org/patch/10804489/
> >
>
> Sorry to move the goal posts on you again Andrea, but Tejun and I talked about
> this some more offline.
>
> We don't want cgroup to become the arbiter of correctness/behavior here. We
> just want it to be isolating things.
>
> For you that means you can drop the per-cgroup flag stuff, and only do the
> priority boosting for multiple sync(2) waiters. That is a real priority
> inversion that needs to be fixed. io.latency and io.max are capable of
> noticing
> that a low priority group is going above their configured limits and putting
> pressure elsewhere accordingly.
Alright, so IIUC that means we just need patch 1/3 for now (with the
per-bdi lock instead of the global lock). If that's the case I'll focus
at that patch then.
>
> Tejun said he'd rather see the sync(2) isolation be done at the namespace
> level.
> That way if you have fs namespacing you are already isolated to your
> namespace.
> If you feel like tackling that then hooray, but that's a separate dragon to
> slay
> so don't feel like you have to right now.
Makes sense. I can take a look and see what I can do after posting the
new patch with the priority inversion fix only.
>
> This way we keep cgroup doing its job, controlling resources. Then we allow
> namespacing to do its thing, isolating resources. Thanks,
>
> Josef
Looks like a good plan to me. Thanks for the update.
-Andrea
