linux-kernel - Re: [PATCH 08/27] mm, vmscan: Simplify the logic deciding whether kswapd sleeps

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Message-ID: <20160616091654.GG1868@techsingularity.net>
Date:	Thu, 16 Jun 2016 10:16:54 +0100
From:	Mel Gorman <mgorman@...hsingularity.net>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>, Rik van Riel <riel@...riel.com>,
	Johannes Weiner <hannes@...xchg.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 08/27] mm, vmscan: Simplify the logic deciding whether
 kswapd sleeps

On Thu, Jun 16, 2016 at 09:30:33AM +0100, Mel Gorman wrote:
> > >@@ -2727,7 +2727,7 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
> > >
> > > 	/* kswapd must be awake if processes are being throttled */
> > > 	if (!wmark_ok && waitqueue_active(&pgdat->kswapd_wait)) {
> > >-		pgdat->classzone_idx = min(pgdat->classzone_idx,
> > >+		pgdat->kswapd_classzone_idx = min(pgdat->kswapd_classzone_idx,
> > > 						(enum zone_type)ZONE_NORMAL);
> > > 		wake_up_interruptible(&pgdat->kswapd_wait);
> > > 	}
> > >@@ -3211,6 +3211,12 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int order,
> > >
> > > 	prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
> > >
> > >+	/* If kswapd has not been woken recently, then full sleep */
> > >+	if (classzone_idx == -1) {
> > >+		classzone_idx = balanced_classzone_idx = MAX_NR_ZONES - 1;
> > >+		goto full_sleep;
> > 
> > This will skip the wakeup_kcompactd() part.
> > 
> 
> I wrestled with this one. I decided to leave it alone on the grounds
> that if kswapd has not been woken recently then compaction efforts also
> have not failed and kcompactd is not required.
> 

And I was wrong. There needs to be a call to wakeup_kcompactd there to
cover the case where there was a single high-order allocation request
that failed and woke kswapd.

-- 
Mel Gorman
SUSE Labs