linux-kernel - [PATCH] mm: slub: make slab_sysfs_init() a late

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Date:   Fri, 30 Sep 2022 12:27:12 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] mm: slub: make slab_sysfs_init() a late_initcall

Currently, slab_sysfs_init() is an __initcall aka device_initcall. It
is rather time-consuming; on my board it takes around 11ms. That's
about 1% of the time budget I have from U-Boot letting go and until
linux must assume responsibility of keeping the external watchdog
happy.

There's no particular reason this would need to run at device_initcall
time, so instead make it a late_initcall to allow vital functionality
to get started a bit sooner.

This actually ends up winning more than just those 11ms, because the
slab caches that get created during other device_initcalls (and before
my watchdog device gets probed) now don't end up doing the somewhat
expensive sysfs_slab_add() themselves. Some example lines (with
initcall_debug set) before/after:

initcall ext4_init_fs+0x0/0x1ac returned 0 after 1386 usecs
initcall journal_init+0x0/0x138 returned 0 after 517 usecs
initcall init_fat_fs+0x0/0x68 returned 0 after 294 usecs

initcall ext4_init_fs+0x0/0x1ac returned 0 after 240 usecs
initcall journal_init+0x0/0x138 returned 0 after 32 usecs
initcall init_fat_fs+0x0/0x68 returned 0 after 18 usecs

Altogether, this means I now get to petting the watchdog around 17ms
sooner. [Of course, the time the other initcalls save is instead spent
in slab_sysfs_init(), which goes from 11ms to 16ms, so there's no
overall change in boot time.]

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---

The numbers certainly suggest that someone might want to look into
making sysfs/kobject/kset perform better. But that would be way more
complicated than this patch, and could not possibly achieve the same
win as getting the sysfs_slab_add() overhead completely out of the
way.

 mm/slub.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 4b98dff9be8e..dade5c84a7bb 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -6070,8 +6070,7 @@ static int __init slab_sysfs_init(void)
 	mutex_unlock(&slab_mutex);
 	return 0;
 }
-
-__initcall(slab_sysfs_init);
+late_initcall(slab_sysfs_init);
 #endif /* CONFIG_SYSFS */

 #if defined(CONFIG_SLUB_DEBUG) && defined(CONFIG_DEBUG_FS)
-- 
2.37.2