On Fri, Dec 13, 2019 at 02:21:49PM -0800, Dennis Zhou wrote:
> 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().
Yeah, the kthread is not guaranteed here.
> 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.
Performance is not my concern here, the addition of bios and blkcg
association is new and there were some integration bugs where I
independently removed early bdev association while the blkg relied on
that. I'm looking for ways to make it less error prone and the kthread
association looks exactly like that so I was curious if it's possible to
use it everywhere. If not, the bdev needs to be found from other
available data.
