Re: move bio cgroup punting into btrfs

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On Wed, Mar 29, 2023 at 09:16:18AM -1000, Tejun Heo wrote:
> We didn't really look deep into adding the support but Chris mentioned that
> raid5/6 are likely to need something similar. Maybe this is because my grasp
> of filesytsems is pretty weak but the pattern doesn't seem unreasonable to
> me. There's some work to be done by a shread kthread and that sometimes can
> fork out IOs which belong to specific cgroups.

Well, in a cgroup aware writeback path we'd always be off much better
to just do the work from a cgroup specific thread instead of bouncing
it around.

> At least in the IO control and direct issue path, punting to just one thread
> hasn't been a practical problem given that when the issuing thread needs to
> be blocked, either the whole device or the cgroup needs to be throttled
> anyway.

I don't think it is a problem per see.  But it is: a) inefficient and
b) complex in terms of code.  So why bounce around between 2, or in case
of writeback 3 threads for a single I/O, instead of making sure your
offload threads are simplify cgroup specific to start with?

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