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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Tue, 22 Mar 2022 08:48:31 +0100
From:   Ingo Molnar <mingo@kernel.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Qian Cai <quic_qiancai@quicinc.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Borislav Petkov <bp@alien8.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Huang, Ying" <ying.huang@intel.com>
Subject: Re: [GIT PULL] scheduler updates for v5.18
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* Ingo Molnar <mingo@kernel.org> wrote:

> 
> * Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 
> > On Mon, Mar 21, 2022 at 6:13 AM Qian Cai <quic_qiancai@quicinc.com> wrote:
> > >
> > > On Mon, Mar 21, 2022 at 11:54:09AM +0100, Ingo Molnar wrote:
> > > > Huang Ying (3):
> > > >       sched/numa-balancing: Move some document to make it consistent with the code
> > > >       sched/numa: Fix NUMA topology for systems with CPU-less nodes
> > > >       sched/numa: Avoid migrating task to CPU-less node
> > >
> > > Linus, I don't think you want to merge this as-is. This will introduce a
> > > kernel crash on arm64 NUMA as mentioned in this thread,
> > 
> > Ok, dropped from my queue. Thanks,
> 
> I've reverted the broken commit & will send another pull request after 
> some testing. Sorry about that!

Ended up using the fix below instead - it's tested already on the affected 
system.

Thanks,

	Ingo

=================>
From: "Huang, Ying" <ying.huang@intel.com>
Date: Tue, 22 Mar 2022 08:39:22 +0100
Subject: [PATCH] sched/numa: Fix boot crash on arm64 systems

Qian Cai reported a boot crash on arm64 systems, caused by:

  0fb3978b0aac ("sched/numa: Fix NUMA topology for systems with CPU-less nodes")

The bug is that node_state() must be supplied a valid node_states[] array index,
but in task_numa_placement() the max_nid search can fail with NUMA_NO_NODE,
which is not a valid index.

Fix it by checking that max_nid is a valid index.

[ mingo: Added changelog. ]

Fixes: 0fb3978b0aac ("sched/numa: Fix NUMA topology for systems with CPU-less nodes")
Reported-by: Qian Cai <quic_qiancai@quicinc.com>
Tested-by: Qian Cai <quic_qiancai@quicinc.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 89d21fda106c..ee0664c9d291 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2437,7 +2437,7 @@ static void task_numa_placement(struct task_struct *p)
 	}
 
 	/* Cannot migrate task to CPU-less node */
-	if (!node_state(max_nid, N_CPU)) {
+	if (max_nid != NUMA_NO_NODE && !node_state(max_nid, N_CPU)) {
 		int near_nid = max_nid;
 		int distance, near_distance = INT_MAX;