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, returning 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 <danielhb...@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 12cbffd3c2e3..4b9df4d645b4 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: -- 2.30.2
