Commit f814bdda authored by Jan Kara's avatar Jan Kara Committed by Jens Axboe
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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: b57d74af ("writeback: track if we're sleeping on progress in balance_dirty_pages()")
Signed-off-by: default avatarJan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20240123175826.21452-1-jack@suse.cz


[axboe: fixup indentation errors]
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent f3c89983

block/blk-wbt.c

+2 −2
Original line number Diff line number Diff line
@@ -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,

include/linux/backing-dev-defs.h

+5 −2
Original line number Diff line number Diff line
@@ -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 */

mm/backing-dev.c

+1 −1
Original line number Diff line number Diff line
@@ -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);

}

mm/page-writeback.c

+1 −1
Original line number Diff line number Diff line
@@ -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;