Re: [PATCH 3/5] writeback: Fix bandwidth estimate for spiky workload

[Hillf Danton <hdanton@sina.com>]

On Mon,  5 Jul 2021 18:23:17 +0200 Jan Kara wrote:
>
>Michael Stapelberg has reported that for workload with short big spikes
>of writes (GCC linker seem to trigger this frequently) the write
>throughput is heavily underestimated and tends to steadily sink until it
>reaches zero. This has rather bad impact on writeback throttling
>(causing stalls). The problem is that writeback throughput estimate gets
>updated at most once per 200 ms. One update happens early after we
>submit pages for writeback (at that point writeout of only small
>fraction of pages is completed and thus observed throughput is tiny).
>Next update happens only during the next write spike (updates happen
>only from inode writeback and dirty throttling code) and if that is
>more than 1s after previous spike, we decide system was idle and just
>ignore whatever was written until this moment.
>
>Fix the problem by making sure writeback throughput estimate is also
>updated shortly after writeback completes to get reasonable estimate of
>throughput for spiky workloads.
>
>Link: https://lore.kernel.org/lkml/20210617095309.3542373-1-stapelberg+li>nux@google.com
>Reported-by: Michael Stapelberg <stapelberg+linux@google.com>
>Signed-off-by: Jan Kara <jack@suse.cz>
>---
> include/linux/backing-dev-defs.h |  1 +
> include/linux/writeback.h        |  1 +
> mm/backing-dev.c                 | 10 ++++++++++
> mm/page-writeback.c              | 32 ++++++++++++++++----------------
> 4 files changed, 28 insertions(+), 16 deletions(-)
>
>diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev>-defs.h
>index 148d889f2f7f..57395f7bb192 100644
>--- a/include/linux/backing-dev-defs.h
>+++ b/include/linux/backing-dev-defs.h
>@@ -143,6 +143,7 @@ struct bdi_writeback {
> 	spinlock_t work_lock;		/* protects work_list & dwork scheduling */
> 	struct list_head work_list;
> 	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 */
>
>diff --git a/include/linux/writeback.h b/include/linux/writeback.h
>index 47cd732e012e..a45e09ed0711 100644
>--- a/include/linux/writeback.h
>+++ b/include/linux/writeback.h
>@@ -379,6 +379,7 @@ int dirty_writeback_centisecs_handler(struct ctl_tabl>e *table, int write,
> void global_dirty_limits(unsigned long *pbackground, unsigned long *pdir>ty);
> unsigned long wb_calc_thresh(struct bdi_writeback *wb, unsigned long thr>esh);
>
>+void wb_update_bandwidth(struct bdi_writeback *wb);
> void balance_dirty_pages_ratelimited(struct address_space *mapping);
> bool wb_over_bg_thresh(struct bdi_writeback *wb);
>
>diff --git a/mm/backing-dev.c b/mm/backing-dev.c
>index 342394ef1e02..9baa59d68110 100644
>--- a/mm/backing-dev.c
>+++ b/mm/backing-dev.c
>@@ -271,6 +271,14 @@ void wb_wakeup_delayed(struct bdi_writeback *wb)
> 	spin_unlock_bh(&wb->work_lock);
> }
>
>+static void wb_update_bandwidth_workfn(struct work_struct *work)
>+{
>+	struct bdi_writeback *wb = container_of(to_delayed_work(work),
>+						struct bdi_writeback, bw_dwork);
>+
>+	wb_update_bandwidth(wb);
>+}
>+
> /*
>  * Initial write bandwidth: 100 MB/s
>  */
>@@ -303,6 +311,7 @@ static int wb_init(struct bdi_writeback *wb, struct b>acking_dev_info *bdi,
> 	spin_lock_init(&wb->work_lock);
> 	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);
>@@ -351,6 +360,7 @@ static void wb_shutdown(struct bdi_writeback *wb)
> 	mod_delayed_work(bdi_wq, &wb->dwork, 0);
> 	flush_delayed_work(&wb->dwork);
> 	WARN_ON(!list_empty(&wb->work_list));
>+	flush_delayed_work(&wb->bw_dwork);
> }
>
> static void wb_exit(struct bdi_writeback *wb)
>diff --git a/mm/page-writeback.c b/mm/page-writeback.c
>index 1fecf8ebadb0..6a99ddca95c0 100644
>--- a/mm/page-writeback.c
>+++ b/mm/page-writeback.c
>@@ -1346,14 +1346,7 @@ static void __wb_update_bandwidth(struct dirty_thr>ottle_control *gdtc,
> 	unsigned long dirtied;
> 	unsigned long written;
>
>-	lockdep_assert_held(&wb->list_lock);
>-
>-	/*
>-	 * rate-limit, only update once every 200ms.
>-	 */
>-	if (elapsed < BANDWIDTH_INTERVAL)
>-		return;

Please leave it as it is if you are not dumping the 200ms rule.

>-
>+	spin_lock(&wb->list_lock);
> 	dirtied = percpu_counter_read(&wb->stat[WB_DIRTIED]);
> 	written = percpu_counter_read(&wb->stat[WB_WRITTEN]);
>
>@@ -1375,15 +1368,14 @@ static void __wb_update_bandwidth(struct dirty_th>rottle_control *gdtc,
> 	wb->dirtied_stamp = dirtied;
> 	wb->written_stamp = written;
> 	wb->bw_time_stamp = now;
>+	spin_unlock(&wb->list_lock);
> }
>
>-static void wb_update_bandwidth(struct bdi_writeback *wb)
>+void wb_update_bandwidth(struct bdi_writeback *wb)
> {
> 	struct dirty_throttle_control gdtc = { GDTC_INIT(wb) };
>
>-	spin_lock(&wb->list_lock);
> 	__wb_update_bandwidth(&gdtc, NULL, false);
>-	spin_unlock(&wb->list_lock);
> }
>
> /* Interval after which we consider wb idle and don't estimate bandwidth> */
>@@ -1728,11 +1720,8 @@ static void balance_dirty_pages(struct bdi_writeba>ck *wb,
> 			wb->dirty_exceeded = 1;
>
> 		if (time_is_before_jiffies(wb->bw_time_stamp +
>-					   BANDWIDTH_INTERVAL)) {
>-			spin_lock(&wb->list_lock);
>+					   BANDWIDTH_INTERVAL))
> 			__wb_update_bandwidth(gdtc, mdtc, true);
>-			spin_unlock(&wb->list_lock);
>-		}
>
> 		/* throttle according to the chosen dtc */
> 		dirty_ratelimit = wb->dirty_ratelimit;
>@@ -2371,7 +2360,13 @@ int do_writepages(struct address_space *mapping, s>truct writeback_control *wbc)
> 		cond_resched();
> 		congestion_wait(BLK_RW_ASYNC, HZ/50);
> 	}
>-	wb_update_bandwidth(wb);
>+	/*
>+	 * Usually few pages are written by now from those we've just submitted
>+	 * but if there's constant writeback being submitted, this makes sure
>+	 * writeback bandwidth is updated once in a while.
>+	 */
>+	if (time_is_before_jiffies(wb->bw_time_stamp + BANDWIDTH_INTERVAL))
>+		wb_update_bandwidth(wb);
> 	return ret;
> }
>
>@@ -2742,6 +2737,11 @@ static void wb_inode_writeback_start(struct bdi_wr>iteback *wb)
> static void wb_inode_writeback_end(struct bdi_writeback *wb)
> {
> 	atomic_dec(&wb->writeback_inodes);
>+	/*
>+	 * Make sure estimate of writeback throughput gets
>+	 * updated after writeback completed.
>+	 */
>+	queue_delayed_work(bdi_wq, &wb->bw_dwork, BANDWIDTH_INTERVAL);
> }

This is a bogus estimate - it does not break the 200ms rule but walks around it
without specifying why 300ms is not good.