blk-wbt: Fix detection of dirty-throttled tasks

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>
Link: https://lore.kernel.org/r/20240123175826.21452-1-jack@suse.cz
[axboe: fixup indentation errors]
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-wbt.c b/block/blk-wbt.c
index 5ba3cd574eacbddc1b92bbaec3d79d81fb66ae7a..0c0e270a82650d9a0c6977931cd8a833b467f520 100644 (file)
--- 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 ae12696ec492c67339409904bb612e9fdc372689..2ad261082bba5f6f0049fa1c642b6ff057f32b5a 100644 (file)
--- 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 1e3447bccdb14d126b3c108fd27ab652b5a3a94f..e039d05304dd9ca52da735962c0ef951fb448ec5 100644 (file)
--- 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 cd4e4ae77c40ae0497efeaa8fb391f6550e51a4b..cc37fa7f336415d880333dc338aaadc236ad809d 100644 (file)
--- 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;