On Thu, Jun 03, 2021 at 12:02:33PM +0200, Jan Kara wrote:
> On Wed 02-06-21 17:55:17, Roman Gushchin wrote:
> > Asynchronously try to release dying cgwbs by switching attached inodes
> > to the bdi's wb. It helps to get rid of per-cgroup writeback
> > structures themselves and of pinned memory and block cgroups, which
> > are significantly larger structures (mostly due to large per-cpu
> > statistics data). This prevents memory waste and helps to avoid
> > different scalability problems caused by large piles of dying cgroups.
> >
> > Reuse the existing mechanism of inode switching used for foreign inode
> > detection. To speed things up batch up to 115 inode switching in a
> > single operation (the maximum number is selected so that the resulting
> > struct inode_switch_wbs_context can fit into 1024 bytes). Because
> > every switching consists of two steps divided by an RCU grace period,
> > it would be too slow without batching. Please note that the whole
> > batch counts as a single operation (when increasing/decreasing
> > isw_nr_in_flight). This allows to keep umounting working (flush the
> > switching queue), however prevents cleanups from consuming the whole
> > switching quota and effectively blocking the frn switching.
> >
> > A cgwb cleanup operation can fail due to different reasons (e.g. not
> > enough memory, the cgwb has an in-flight/pending io, an attached inode
> > in a wrong state, etc). In this case the next scheduled cleanup will
> > make a new attempt. An attempt is made each time a new cgwb is offlined
> > (in other words a memcg and/or a blkcg is deleted by a user). In the
> > future an additional attempt scheduled by a timer can be implemented.
> >
> > Signed-off-by: Roman Gushchin <guro@xxxxxx>
>
> I think we are getting close :). Some comments are below.
Great! Thank for reviewing the code!
>
> > ---
> > fs/fs-writeback.c | 68 ++++++++++++++++++++++++++++++++
> > include/linux/backing-dev-defs.h | 1 +
> > include/linux/writeback.h | 1 +
> > mm/backing-dev.c | 58 ++++++++++++++++++++++++++-
> > 4 files changed, 126 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > index 49d7b23a7cfe..e8517ad677eb 100644
> > --- a/fs/fs-writeback.c
> > +++ b/fs/fs-writeback.c
> > @@ -225,6 +225,8 @@ void wb_wait_for_completion(struct wb_completion *done)
> > /* one round can affect upto 5 slots */
> > #define WB_FRN_MAX_IN_FLIGHT 1024 /* don't queue too many concurrently */
> >
> > +#define WB_MAX_INODES_PER_ISW 116 /* maximum inodes per isw */
> > +
>
> Why this number? Please add an explanation here...
Added.
>
> > static atomic_t isw_nr_in_flight = ATOMIC_INIT(0);
> > static struct workqueue_struct *isw_wq;
> >
> > @@ -552,6 +554,72 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
> > kfree(isw);
> > }
> >
> > +/**
> > + * cleanup_offline_cgwb - detach associated inodes
> > + * @wb: target wb
> > + *
> > + * Switch all inodes attached to @wb to the bdi's root wb in order to eventually
> > + * release the dying @wb. Returns %true if not all inodes were switched and
> > + * the function has to be restarted.
> > + */
> > +bool cleanup_offline_cgwb(struct bdi_writeback *wb)
> > +{
> > + struct inode_switch_wbs_context *isw;
> > + struct inode *inode;
> > + int nr;
> > + bool restart = false;
> > +
> > + isw = kzalloc(sizeof(*isw) + WB_MAX_INODES_PER_ISW *
> > + sizeof(struct inode *), GFP_KERNEL);
> > + if (!isw)
> > + return restart;
> > +
> > + /* no need to call wb_get() here: bdi's root wb is not refcounted */
> > + isw->new_wb = &wb->bdi->wb;
> > +
> > + nr = 0;
> > + spin_lock(&wb->list_lock);
> > + list_for_each_entry(inode, &wb->b_attached, i_io_list) {
> > + spin_lock(&inode->i_lock);
> > + if (!(inode->i_sb->s_flags & SB_ACTIVE) ||
> > + inode->i_state & (I_WB_SWITCH | I_FREEING) ||
> > + inode_to_wb(inode) == isw->new_wb) {
> > + spin_unlock(&inode->i_lock);
> > + continue;
> > + }
> > + inode->i_state |= I_WB_SWITCH;
> > + __iget(inode);
> > + spin_unlock(&inode->i_lock);
>
> This hunk is identical with the one in inode_switch_wbs(). Maybe create a
> helper for it like inode_prepare_wb_switch() or something like that. Also
> we need to check for I_WILL_FREE flag as well as I_FREEING (see the code in
> iput_final()) - that's actually a bug in inode_switch_wbs() as well so
> probably a separate fix for that should come earlier in the series.
Good point, added in v7.
>
> > +
> > + isw->inodes[nr++] = inode;
>
> At first it seemed a bit silly to allocate an array of inode pointers when
> we have them in the list. But after some thought I agree that dealing with
> other switching being triggered from other sources in parallel would be
> really difficult so your decision makes sense. Just maybe add an
> explanation in a comment somewhere about this design decision.
Added in v7.
>
> > +
> > + if (nr >= WB_MAX_INODES_PER_ISW - 1) {
> > + restart = true;
> > + break;
> > + }
> > + }
> > + spin_unlock(&wb->list_lock);
>
> ...
>
> > +static void cleanup_offline_cgwbs_workfn(struct work_struct *work)
> > +{
> > + struct bdi_writeback *wb;
> > + LIST_HEAD(processed);
> > +
> > + spin_lock_irq(&cgwb_lock);
> > +
> > + while (!list_empty(&offline_cgwbs)) {
> > + wb = list_first_entry(&offline_cgwbs, struct bdi_writeback,
> > + offline_node);
> > + list_move(&wb->offline_node, &processed);
> > +
> > + if (wb_has_dirty_io(wb))
> > + continue;
>
> Maybe explain in a comment why skipping wbs with dirty inodes is fine?
> Because honestly, I'm not sure... I guess the rationale is that inodes
> should get cleaned eventually and if they are getting redirtied, they will
> be switched to another wb anyway?
The main rationale here is that the deletion of a memory/blkcg cgroup by a user
shouldn't affect the io distribution. In other words, the remaining io shouldn't
be performed faster than it could be finished had the cgroup remain existing.
