linux-kernel - Re: [PATCH 1/1] mm: vmscan: Reduce throttling due to a failure to make progress

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Message-ID: <1c1835271e0ea093dd169d19038b477cf8563c32.camel@gmx.de>
Date:   Sun, 28 Nov 2021 19:38:37 +0100
From:   Mike Galbraith <efault@....de>
To:     Alexey Avramov <hakavlad@...ox.lv>,
        Mel Gorman <mgorman@...hsingularity.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Rik van Riel <riel@...riel.com>,
        Darrick Wong <djwong@...nel.org>, regressions@...ts.linux.dev,
        Linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] mm: vmscan: Reduce throttling due to a failure to
 make progress

On Sun, 2021-11-28 at 13:35 +0100, Mike Galbraith wrote:
>
> Bandaid likely just break the writeback bits all to pieces, but that's
> ok, if breakage makes things work better, they need more breaking :)

@@ -1048,6 +1050,10 @@ void reclaim_throttle(pg_data_t *pgdat,
 	 */
 	switch(reason) {
 	case VMSCAN_THROTTLE_WRITEBACK:
+		if (!async_bdi_congested()) {
+			cond_resched();
+			return;
+		}

And indeed, that's the only THROTTLE_WRITEBACK path I've seen taken.
Not pulling the plug made no difference to the tail /dev/zero test,
leaving only the bits I swiped from defunct wait_iff_congested() as
behavior delta cause.  Actually sleeping there apparently stings.

	-Mike