linux-kernel - [PATCH] mm: initialize deferred pages with interrupts enabled

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Date:   Wed,  1 Apr 2020 15:32:38 -0400
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        mhocko@...e.com, linux-mm@...ck.org, dan.j.williams@...el.com,
        shile.zhang@...ux.alibaba.com, daniel.m.jordan@...cle.com,
        pasha.tatashin@...een.com, ktkhai@...tuozzo.com, david@...hat.com,
        jmorris@...ei.org, sashal@...nel.org
Subject: [PATCH] mm: initialize deferred pages with interrupts enabled

Initializing struct pages is a long task and keeping interrupts disabled
for the duration of this operation introduces a number of problems.

1. jiffies are not updated for long period of time, and thus incorrect time
   is reported. See proposed solution and discussion here:
   lkml/20200311123848.118638-1-shile.zhang@...ux.alibaba.com
2. It prevents farther improving deferred page initialization by allowing
   inter-node multi-threading.

We are keeping interrupts disabled to solve a rather theoretical problem
that was never observed in real world (See 3a2d7fa8a3d5).

Lets keep interrupts enabled. In case we ever encounter a scenario where
an interrupt thread wants to allocate large amount of memory this early in
boot we can deal with that by growing zone (see deferred_grow_zone()) by
the needed amount before starting deferred_init_memmap() threads.

Before:
[    1.232459] node 0 initialised, 12058412 pages in 1ms

After:
[    1.632580] node 0 initialised, 12051227 pages in 436ms

Signed-off-by: Pavel Tatashin <pasha.tatashin@...een.com>
---
 mm/page_alloc.c | 21 +++++++--------------
 1 file changed, 7 insertions(+), 14 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3c4eb750a199..4498a13b372d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1792,6 +1792,13 @@ static int __init deferred_init_memmap(void *data)
 	BUG_ON(pgdat->first_deferred_pfn > pgdat_end_pfn(pgdat));
 	pgdat->first_deferred_pfn = ULONG_MAX;
 
+	/*
+	 * Once we unlock here, the zone cannot be grown anymore, thus if an
+	 * interrupt thread must allocate this early in boot, zone must be
+	 * pre-grown prior to start of deferred page initialization.
+	 */
+	pgdat_resize_unlock(pgdat, &flags);
+
 	/* Only the highest zone is deferred so find it */
 	for (zid = 0; zid < MAX_NR_ZONES; zid++) {
 		zone = pgdat->node_zones + zid;
@@ -1812,8 +1819,6 @@ static int __init deferred_init_memmap(void *data)
 	while (spfn < epfn)
 		nr_pages += deferred_init_maxorder(&i, zone, &spfn, &epfn);
 zone_empty:
-	pgdat_resize_unlock(pgdat, &flags);
-
 	/* Sanity check that the next zone really is unpopulated */
 	WARN_ON(++zid < MAX_NR_ZONES && populated_zone(++zone));
 
@@ -1854,18 +1859,6 @@ deferred_grow_zone(struct zone *zone, unsigned int order)
 		return false;
 
 	pgdat_resize_lock(pgdat, &flags);
-
-	/*
-	 * If deferred pages have been initialized while we were waiting for
-	 * the lock, return true, as the zone was grown.  The caller will retry
-	 * this zone.  We won't return to this function since the caller also
-	 * has this static branch.
-	 */
-	if (!static_branch_unlikely(&deferred_pages)) {
-		pgdat_resize_unlock(pgdat, &flags);
-		return true;
-	}
-
 	/*
 	 * If someone grew this zone while we were waiting for spinlock, return
 	 * true, as there might be enough pages already.
-- 
2.17.1