On Fri, Dec 13, 2019 at 01:24:01PM +0100, David Sterba wrote:
> On Thu, Dec 12, 2019 at 10:19:34AM -0800, Dennis Zhou wrote:
> > From a0569aebde08e31e994c92d0b70befb84f7f5563 Mon Sep 17 00:00:00 2001
> > From: Dennis Zhou <dennis@xxxxxxxxxx>
> > Date: Wed, 11 Dec 2019 15:20:15 -0800
> >
> > Bio attribution is handled at bio_set_dev() as once we have a device, we
> > have a corresponding request_queue and then can derive the current css.
> > In special cases, we want to attribute to bio to someone else. This can
> > be done by calling bio_associate_blkg_from_css() or
> > kthread_associate_blkcg() depending on the scenario. Btrfs does this for
> > compressed writeback as they are handled by kworkers, so the latter can
> > be done here.
> >
> > Commit 1a41802701ec ("btrfs: drop bio_set_dev where not needed") removes
> > early bio_set_dev() calls prior to submit_stripe_bio(). This breaks the
> > above assumption that we'll have a request_queue when we are doing
> > association. To fix this, switch to using kthread_associate_blkcg().
>
> Can be kthread_associate_blkcg used also for submit_extent_page that
> calls bio_associate_blkg_from_css indirectly when initializing wbc?
>
> 2996 bio_set_dev(bio, bdev);
> 2997 wbc_init_bio(wbc, bio);
> 2998 wbc_account_cgroup_owner(wbc, page, page_size);
>
> wbc_init_bio:
>
> if (wbc)
> bio_associate_blkg_from_css();
Correct me if I'm wrong, but I don't think submit_extent_page() is only
called from kthread contexts. So, we wouldn't be able to rely on
kthread_associate_blkcg().
I can think about how to make wbc better for association in general, but
it's a percpu decrement and increment so it shouldn't really be much in
overhead.
Thanks,
Dennis
