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Subject: Re: [rfc/patch] mm/slub: restore/expand unfreeze_partials() local
 exclusion scope
From:   Mike Galbraith <efault@gmx.de>
To:     Vlastimil Babka <vbabka@suse.cz>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>
Cc:     linux-rt-users@vger.kernel.org,
        Mel Gorman <mgorman@techsingularity.net>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date:   Wed, 21 Jul 2021 11:33:41 +0200
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On Wed, 2021-07-21 at 10:44 +0200, Vlastimil Babka wrote:
>
> So this doesn't look like our put_cpu_partial() preempted a
> __slab_alloc() on the same cpu, right?

No, likely it was the one preempted by someone long gone, but we'll
never know without setting a trap.

> BTW did my ugly patch work?

Nope.  I guess you missed my reporting it to have been a -ENOBOOT, and
that cutting it in half, ie snagging only __slab_free() does boot, and
seems to cure all of the RT fireworks.

(chainsaw noises...)

=2D--
 mm/slub.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

=2D-- a/mm/slub.c
+++ b/mm/slub.c
@@ -2551,6 +2551,8 @@ static void put_cpu_partial(struct kmem_
 	int pobjects;

 	slub_get_cpu_ptr(s->cpu_slab);
+	if (IS_ENABLED(CONFIG_PREEMPT_RT) && drain)
+		local_lock(&s->cpu_slab->lock);
 	do {
 		pages =3D 0;
 		pobjects =3D 0;
@@ -2564,7 +2566,13 @@ static void put_cpu_partial(struct kmem_
 				 * partial array is full. Move the existing
 				 * set to the per node partial list.
 				 */
-				unfreeze_partials(s);
+				this_cpu_write(s->cpu_slab->partial, NULL);
+				if (IS_ENABLED(CONFIG_PREEMPT_RT) && drain)
+					local_unlock(&s->cpu_slab->lock);
+				__unfreeze_partials(s, oldpage);
+				if (IS_ENABLED(CONFIG_PREEMPT_RT) && drain)
+					local_lock(&s->cpu_slab->lock);
+
 				oldpage =3D NULL;
 				pobjects =3D 0;
 				pages =3D 0;
@@ -2581,6 +2589,8 @@ static void put_cpu_partial(struct kmem_

 	} while (this_cpu_cmpxchg(s->cpu_slab->partial, oldpage, page)
 								!=3D oldpage);
+	if (IS_ENABLED(CONFIG_PREEMPT_RT) && drain)
+		local_unlock(&s->cpu_slab->lock);
 	slub_put_cpu_ptr(s->cpu_slab);
 #endif	/* CONFIG_SLUB_CPU_PARTIAL */
 }