From: Daniel Henrique Barboza <danielhb413@gmail.com>
To: linuxppc-dev@lists.ozlabs.org
Cc: Daniel Henrique Barboza <danielhb413@gmail.com>
Subject: [PATCH v2 1/1] hotplug-cpu.c: show 'last online CPU' error in dlpar_cpu_offline()
Date: Tue, 23 Mar 2021 17:50:56 -0300 [thread overview]
Message-ID: <20210323205056.52768-2-danielhb413@gmail.com> (raw)
In-Reply-To: <20210323205056.52768-1-danielhb413@gmail.com>
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. See [1] for an example.
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
[1] https://bugzilla.redhat.com/1911414
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
---
arch/powerpc/platforms/pseries/hotplug-cpu.c | 13 +++++++++++++
1 file changed, 13 insertions(+)
diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
index 12cbffd3c2e3..3ac7e904385c 100644
--- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
+++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
@@ -271,6 +271,18 @@ 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 +295,7 @@ static int dlpar_offline_cpu(struct device_node *dn)
thread);
}
}
+out_unlock:
cpu_maps_update_done();
out:
--
2.30.2
next prev parent reply other threads:[~2021-03-23 20:51 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-23 20:50 [PATCH v2 0/1] show 'last online CPU' error in dlpar_cpu_offline() Daniel Henrique Barboza
2021-03-23 20:50 ` Daniel Henrique Barboza [this message]
2021-03-26 5:24 ` [PATCH v2 1/1] hotplug-cpu.c: " Daniel Axtens
2021-03-26 14:08 ` Daniel Henrique Barboza
2021-03-31 1:09 ` [PATCH v2 0/1] " Michael Ellerman
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