[PATCH v4 2/2] drivers/hv: add CPU offlining support

Hamza Mahfooz
2025-01-16T13:41:51Z

Currently, it is tedious to offline CPUs. Since, most CPUs will have
vmbus channels attached to them that a user would have to manually
rebind elsewhere. So, as made mention of in
commit d570aec0f2154 ("Drivers: hv: vmbus: Synchronize init_vp_index()
vs. CPU hotplug"), rebind channels associated with CPUs that a user is
trying to offline to a new "randomly" selected CPU.

Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Michael Kelley <mhklinux@outlook.com>
Cc: Wei Liu <wei.liu@kernel.org>
Signed-off-by: Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>
---
v2: remove cpus_read_{un,}lock() from hv_pick_new_cpu() and add
lockdep_assert_cpus_held().

v3: use for_each_cpu_wrap() in hv_pick_new_cpu().

v4: store get_random_u32_below() in start.
---
drivers/hv/hv.c | 65 +++++++++++++++++++++++++++++++++++++------------
1 file changed, 50 insertions(+), 15 deletions(-)

diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c
index 36d9ba097ff5..4388c0030a59 100644
--- a/drivers/hv/hv.c
+++ b/drivers/hv/hv.c
@@ -433,13 +433,48 @@ static bool hv_synic_event_pending(void)
return pending;
}

+static int hv_pick_new_cpu(struct vmbus_channel *channel,
+ unsigned int current_cpu)
+{
+ int ret = 0;
+ int start;
+ int cpu;
+
+ lockdep_assert_cpus_held();
+ lockdep_assert_held(&vmbus_connection.channel_mutex);
+
+ /*
+ * We can't assume that the relevant interrupts will be sent before
+ * the cpu is offlined on older versions of hyperv.
+ */
+ if (vmbus_proto_version < VERSION_WIN10_V5_3)
+ return -EBUSY;
+
+ start = get_random_u32_below(nr_cpu_ids);
+
+ for_each_cpu_wrap(cpu, cpu_online_mask, start) {
+ if (cpu == current_cpu || cpu == VMBUS_CONNECT_CPU)
+ continue;
+
+ ret = vmbus_channel_set_cpu(channel, cpu);
+
+ if (!ret)
+ break;
+ }
+
+ if (ret)
+ ret = vmbus_channel_set_cpu(channel, VMBUS_CONNECT_CPU);
+
+ return ret;
+}
+
/*
* hv_synic_cleanup - Cleanup routine for hv_synic_init().
*/
int hv_synic_cleanup(unsigned int cpu)
{
struct vmbus_channel *channel, *sc;
- bool channel_found = false;
+ int ret = 0;

if (vmbus_connection.conn_state != CONNECTED)
goto always_cleanup;
@@ -456,31 +491,31 @@ int hv_synic_cleanup(unsigned int cpu)

/*
* Search for channels which are bound to the CPU we're about to
- * cleanup. In case we find one and vmbus is still connected, we
- * fail; this will effectively prevent CPU offlining.
- *
- * TODO: Re-bind the channels to different CPUs.
+ * cleanup.
*/
mutex_lock(&vmbus_connection.channel_mutex);
list_for_each_entry(channel, &vmbus_connection.chn_list, listentry) {
if (channel->target_cpu == cpu) {
- channel_found = true;
- break;
+ ret = hv_pick_new_cpu(channel, cpu);
+
+ if (ret) {
+ mutex_unlock(&vmbus_connection.channel_mutex);
+ return ret;
+ }
}
list_for_each_entry(sc, &channel->sc_list, sc_list) {
if (sc->target_cpu == cpu) {
- channel_found = true;
- break;
+ ret = hv_pick_new_cpu(channel, cpu);
+
+ if (ret) {
+ mutex_unlock(&vmbus_connection.channel_mutex);
+ return ret;
+ }
}
}
- if (channel_found)
- break;
}
mutex_unlock(&vmbus_connection.channel_mutex);

- if (channel_found)
- return -EBUSY;
-
/*
* channel_found == false means that any channels that were previously
* assigned to the CPU have been reassigned elsewhere with a call of
@@ -497,5 +532,5 @@ int hv_synic_cleanup(unsigned int cpu)

hv_synic_disable_regs(cpu);

- return 0;
+ return ret;
}
--
2.47.1