Message-ID: <252cd5acd9bf6588ec87ce02884925c737b6a8b7.camel@gmx.de>
Subject: Re: [PATCH 1/1] mm: vmscan: Reduce throttling due to a failure to
 make progress
From:   Mike Galbraith <efault@gmx.de>
To:     Alexey Avramov <hakavlad@inbox.lv>,
        Mel Gorman <mgorman@techsingularity.net>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Rik van Riel <riel@surriel.com>,
        Darrick Wong <djwong@kernel.org>, regressions@lists.linux.dev,
        Linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
Date:   Sun, 28 Nov 2021 11:00:59 +0100
In-Reply-To: <20211128042635.543a2d04@mail.inbox.lv>
References: <20211125151853.8540-1-mgorman@techsingularity.net>
         <20211127011246.7a8ac7b8@mail.inbox.lv>
         <20211126165211.GL3366@techsingularity.net>
         <20211128042635.543a2d04@mail.inbox.lv>
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On Sun, 2021-11-28 at 04:26 +0900, Alexey Avramov wrote:
> I will present the results of the new tests here.
>
> TLDR;
> =3D=3D=3D=3D=3D
> No one Mel's patch doesn't prevent stalls in my tests.

Seems there may be a problem with the THROTTLE_WRITEBACK bits..

> $ for i in {1..10}; do tail /dev/zero; done
> -- 1. with noswap

..because the bandaid below (made of 8cd7c588 shards) on top of Mel's
last pulled that one-liner's very pointy fangs.

=2D--
 mm/backing-dev.c |    5 +++++
 mm/vmscan.c      |    8 +++++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

=2D-- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -1055,3 +1055,8 @@ long congestion_wait(int sync, long time
 	return ret;
 }
 EXPORT_SYMBOL(congestion_wait);
+
+int async_bdi_congested(void)
+{
+	return atomic_read(&nr_wb_congested[BLK_RW_ASYNC]) !=3D 0;
+}
=2D-- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1021,6 +1021,8 @@ static void handle_write_error(struct ad
 	unlock_page(page);
 }

+extern int async_bdi_congested(void);
+
 void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason=
)
 {
 	wait_queue_head_t *wqh =3D &pgdat->reclaim_wait[reason];
@@ -1048,6 +1050,10 @@ void reclaim_throttle(pg_data_t *pgdat,
 	 */
 	switch(reason) {
 	case VMSCAN_THROTTLE_WRITEBACK:
+		if (!async_bdi_congested()) {
+			cond_resched();
+			return;
+		}
 		timeout =3D HZ/10;

 		if (atomic_inc_return(&pgdat->nr_writeback_throttled) =3D=3D 1) {
@@ -1079,7 +1085,7 @@ void reclaim_throttle(pg_data_t *pgdat,
 	}

 	prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
-	ret =3D schedule_timeout(timeout);
+	ret =3D io_schedule_timeout(timeout);
 	finish_wait(wqh, &wait);

 	if (reason =3D=3D VMSCAN_THROTTLE_WRITEBACK)