From d19b3ad02c2d1a9a697b7059e32fa2d97a420b15 Mon Sep 17 00:00:00 2001
From: Daniel Henrique Barboza <danielhb413@gmail.com>
Date: Tue, 23 Mar 2021 17:50:56 -0300
Subject: [PATCH] powerpc/pseries/hotplug-cpu: Show 'last online CPU' error in
dlpar_cpu_offline()
One of the reasons that dlpar_cpu_offline can fail is when attempting to
offline the last online CPU of the kernel. This can be observed in a
pseries QEMU guest that has hotplugged CPUs. If the user offlines all
other CPUs of the guest, and a hotplugged CPU is now the last online
CPU, trying to reclaim it will fail.
The current error message in this situation returns rc with -EBUSY and a
generic explanation, e.g.:
pseries-hotplug-cpu: Failed to offline CPU PowerPC,POWER9, rc: -16
EBUSY can be caused by other conditions, such as cpu_hotplug_disable
being true. Throwing a more specific error message for this case,
instead of just "Failed to offline CPU", makes it clearer that the error
is in fact a known error situation instead of other generic/unknown
cause.
This patch adds a 'last online' check in dlpar_cpu_offline() to catch
the 'last online CPU' offline error, eturning a more informative error
message:
pseries-hotplug-cpu: Unable to remove last online CPU PowerPC,POWER9
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210323205056.52768-2-danielhb413@gmail.com
---
arch/powerpc/platforms/pseries/hotplug-cpu.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)
diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
index 12cbffd3c2e32c..ec478f8a98ff26 100644
--- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
+++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
@@ -271,6 +271,19 @@ static int dlpar_offline_cpu(struct device_node *dn)
if (!cpu_online(cpu))
break;
+ /*
+ * device_offline() will return -EBUSY (via cpu_down()) if there
+ * is only one CPU left. Check it here to fail earlier and with a
+ * more informative error message, while also retaining the
+ * cpu_add_remove_lock to be sure that no CPUs are being
+ * online/offlined during this check.
+ */
+ if (num_online_cpus() == 1) {
+ pr_warn("Unable to remove last online CPU %pOFn\n", dn);
+ rc = -EBUSY;
+ goto out_unlock;
+ }
+
cpu_maps_update_done();
rc = device_offline(get_cpu_device(cpu));
if (rc)
@@ -283,6 +296,7 @@ static int dlpar_offline_cpu(struct device_node *dn)
thread);
}
}
+out_unlock:
cpu_maps_update_done();
out:
--
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