Re: [PATCH RFC 10/22] block, bfq: add full hierarchical scheduling and cgroups support

[Tejun Heo]

Hello,

On Mon, Feb 01, 2016 at 11:12:46PM +0100, Paolo Valente wrote:
> From: Arianna Avanzini <avanzini.arianna@xxxxxxxxx>
> 
> Complete support for full hierarchical scheduling, with a cgroups
> interface. The name of the added policy is bfq.
> 
> Weights can be assigned explicitly to groups and processes through the
> cgroups interface, differently from what happens, for single
> processes, if the cgroups interface is not used (as explained in the
> description of the previous patch). In particular, since each node has
> a full scheduler, each group can be assigned its own weight.

* It'd be great if how cgroup support is achieved is better
  documented.

* How's writeback handled?

* After all patches are applied, both CONFIG_BFQ_GROUP_IOSCHED and
  CONFIG_CFQ_GROUP_IOSCHED exist.

* The default weight and weight range don't seem to follow the defined
  interface on the v2 hierarchy.  The default value should be 100.

* With all patches applied, booting triggers a RCU context warning.
  Please build with lockdep and RCU debugging turned on and fix the
  issue.

* I was testing on the v2 hierarchy with two top-level cgroups one
  hosting sequential workload and the other completely random.  While
  they eventually converged to a reasonable state, starting up the
  sequential workload while the random workload was running was
  extremely slow.  It crawled for quite a while.

* And "echo 100 > io.weight" hung the writing process.

Thanks.

-- 
tejun