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

[Paolo Valente]

Il giorno 11/feb/2016, alle ore 23:28, Tejun Heo <tj@xxxxxxxxxx> ha scritto:

> 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.
> 

ok, I will do it.

> * How's writeback handled?
> 

If I understood correctly your question, then the answer is that there is no special/automatic handling of writeback queues. Thus, unless the user explicitly inserts flushing threads in some groups and plays with the weights of these groups, these threads will just get the default weight, and thus the default treatment for queues in the root group. IOW, no privileges. The motivation is that these threads serve asynchronous requests, i.e., requests that do not cause any delay to the processes that issue them. Therefore, apart from higher-level considerations on vm page-flushing pressure, there is basically no point in privileging writeback I/O with respect to other types of possibly time-sensitive I/O.


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

Sorry, thanks.

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

Sorry again, I will fix it.

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

Ok, I will do it.

> * 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.
> 

This is definitely a bug. Could you please (maybe privately?) send me the exact commands/script you have used?

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

I’m not sure I understood exactly which process you are referring to, but I guess I will probably understand it from the commands I have asked you to share.

Thanks,
Paolo

> Thanks.
> 
> -- 
> tejun