On Tue 23-01-24 18:58:26, Jan Kara wrote:

The detection of dirty-throttled tasks in blk-wbt has been subtly broken since its beginning in 2016. Namely if we are doing cgroup writeback and the throttled task is not in the root cgroup, balance_dirty_pages() will set dirty_sleep for the non-root bdi_writeback structure. However blk-wbt checks dirty_sleep only in the root cgroup bdi_writeback structure. Thus detection of recently throttled tasks is not working in this case (we noticed this when we switched to cgroup v2 and suddently writeback was slow).

Since blk-wbt has no easy way to get to proper bdi_writeback and furthermore its intention has always been to work on the whole device rather than on individual cgroups, just move the dirty_sleep timestamp from bdi_writeback to backing_dev_info. That fixes the checking for recently throttled task and saves memory for everybody as a bonus.

CC: stable@vger.kernel.org Fixes: b57d74aff9ab ("writeback: track if we're sleeping on progress in balance_dirty_pages()") Signed-off-by: Jan Kara jack@suse.cz

Ping Jens?

Honza

---

block/blk-wbt.c | 4 ++-- include/linux/backing-dev-defs.h | 7 +++++-- mm/backing-dev.c | 2 +- mm/page-writeback.c | 2 +- 4 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/block/blk-wbt.c b/block/blk-wbt.c index 5ba3cd574eac..0c0e270a8265 100644 --- a/block/blk-wbt.c +++ b/block/blk-wbt.c @@ -163,9 +163,9 @@ static void wb_timestamp(struct rq_wb *rwb, unsigned long *var) */ static bool wb_recent_wait(struct rq_wb *rwb) {

• struct bdi_writeback *wb = &rwb->rqos.disk->bdi->wb;

• struct backing_dev_info *bdi = rwb->rqos.disk->bdi;

• return time_before(jiffies, wb->dirty_sleep + HZ);

• return time_before(jiffies, bdi->last_bdp_sleep + HZ);

} static inline struct rq_wait *get_rq_wait(struct rq_wb *rwb, diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h index ae12696ec492..ad17739a2e72 100644 --- a/include/linux/backing-dev-defs.h +++ b/include/linux/backing-dev-defs.h @@ -141,8 +141,6 @@ struct bdi_writeback { struct delayed_work dwork; /* work item used for writeback */ struct delayed_work bw_dwork; /* work item used for bandwidth estimate */

• unsigned long dirty_sleep; /* last wait */
• struct list_head bdi_node; /* anchored at bdi->wb_list */

#ifdef CONFIG_CGROUP_WRITEBACK @@ -179,6 +177,11 @@ struct backing_dev_info { * any dirty wbs, which is depended upon by bdi_has_dirty(). */ atomic_long_t tot_write_bandwidth;

• /*

• * Jiffies when last process was dirty throttled on this bdi. Used by
  

• * blk-wbt.
  

• */
• unsigned long last_bdp_sleep;

struct bdi_writeback wb; /* the root writeback info for this bdi */ struct list_head wb_list; /* list of all wbs */ diff --git a/mm/backing-dev.c b/mm/backing-dev.c index 1e3447bccdb1..e039d05304dd 100644 --- a/mm/backing-dev.c +++ b/mm/backing-dev.c @@ -436,7 +436,6 @@ static int wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi, INIT_LIST_HEAD(&wb->work_list); INIT_DELAYED_WORK(&wb->dwork, wb_workfn); INIT_DELAYED_WORK(&wb->bw_dwork, wb_update_bandwidth_workfn);

• wb->dirty_sleep = jiffies;

err = fprop_local_init_percpu(&wb->completions, gfp); if (err) @@ -921,6 +920,7 @@ int bdi_init(struct backing_dev_info *bdi) INIT_LIST_HEAD(&bdi->bdi_list); INIT_LIST_HEAD(&bdi->wb_list); init_waitqueue_head(&bdi->wb_waitq);

• bdi->last_bdp_sleep = jiffies;

return cgwb_bdi_init(bdi); } diff --git a/mm/page-writeback.c b/mm/page-writeback.c index cd4e4ae77c40..cc37fa7f3364 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -1921,7 +1921,7 @@ static int balance_dirty_pages(struct bdi_writeback *wb, break; } __set_current_state(TASK_KILLABLE);

• wb->dirty_sleep = now;
  

• bdi->last_bdp_sleep = jiffies;
  

  io_schedule_timeout(pause);

current->dirty_paused_when = now + pause; -- 2.35.3