Re: [PATCH v2 0/3] blkcg: sync() isolation

[Josef Bacik]

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.

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.

This way we keep cgroup doing its job, controlling resources.  Then we allow
namespacing to do its thing, isolating resources.  Thanks,

Josef