Message-ID: <1c1835271e0ea093dd169d19038b477cf8563c32.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 19:38:37 +0100
In-Reply-To: <dd39029d93bb4de5ed485b5d4181fc19d4c0c4f0.camel@gmx.de>
References: <20211125151853.8540-1-mgorman@techsingularity.net>
         <20211127011246.7a8ac7b8@mail.inbox.lv>
         <20211126165211.GL3366@techsingularity.net>
         <20211128042635.543a2d04@mail.inbox.lv>
         <252cd5acd9bf6588ec87ce02884925c737b6a8b7.camel@gmx.de>
         <941f378e1ea2b32cac0adee1e81637ab6d001f1e.camel@gmx.de>
         <dd39029d93bb4de5ed485b5d4181fc19d4c0c4f0.camel@gmx.de>
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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