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.
> ---
> 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...
> 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.
> +
> + 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.
> +
> + 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?
> +
> + if (!wb_tryget(wb))
> + continue;
> +
> + spin_unlock_irq(&cgwb_lock);
> + while ((cleanup_offline_cgwb(wb)))
> + cond_resched();
> + spin_lock_irq(&cgwb_lock);
> +
> + wb_put(wb);
> + }
> +
> + if (!list_empty(&processed))
> + list_splice_tail(&processed, &offline_cgwbs);
> +
> + spin_unlock_irq(&cgwb_lock);
> +}
> +
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
