Re: [PATCH 8/8] cfq-iosched: charge async IOs to the appropriate blkcg's instead of the root

[Jeff Moyer <jmoyer@redhat.com>]

Tejun Heo <tj@kernel.org> writes:

> Up until now, all async IOs were queued to async queues which are
> shared across the whole request_queue, which means that blkcg resource
> control is completely void on async IOs including all writeback IOs.
> It was done this way because writeback didn't support writeback and
> there was no way of telling which writeback IO belonged to which
> cgroup; however, writeback recently became cgroup aware and writeback
> bio's are sent down properly tagged with the blkcg's to charge them
> against.
>
> This patch makes async cfq_queues per-cfq_cgroup instead of
> per-cfq_data so that each async IO is charged to the blkcg that it was
> tagged for instead of unconditionally attributing it to root.
>
> * cfq_data->async_cfqq and ->async_idle_cfqq are moved to cfq_group
>   and alloc / destroy paths are updated accordingly.
>
> * cfq_link_cfqq_cfqg() no longer overrides @cfqg to root for async
>   queues.
>
> * check_blkcg_changed() now also invalidates async queues as they no
>   longer stay the same across cgroups.
>
> After this patch, cfq's proportional IO control through blkio.weight
> works correctly when cgroup writeback is in use.
>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Vivek Goyal <vgoyal@redhat.com>
> Cc: Arianna Avanzini <avanzini.arianna@gmail.com>

Reviewed-by: Jeff Moyer <jmoyer@redhat.com>