linux-kernel - [PATCH v2 2/2] drivers/hv: add CPU offlining support

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Message-ID: <20250110215951.175514-2-hamzamahfooz@linux.microsoft.com>
Date: Fri, 10 Jan 2025 16:59:50 -0500
From: Hamza Mahfooz <hamzamahfooz@...ux.microsoft.com>
To: linux-hyperv@...r.kernel.org
Cc: Hamza Mahfooz <hamzamahfooz@...ux.microsoft.com>,
	Boqun Feng <boqun.feng@...il.com>,
	Wei Liu <wei.liu@...nel.org>,
	"K. Y. Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Dexuan Cui <decui@...rosoft.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2 2/2] drivers/hv: add CPU offlining support

Currently, it is effectively impossible to offline CPUs. Since, most
CPUs will have vmbus channels attached to them. 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@...il.com>
Cc: Wei Liu <wei.liu@...nel.org>
Signed-off-by: Hamza Mahfooz <hamzamahfooz@...ux.microsoft.com>
---
v2: remove cpus_read_{un,}lock() from hv_pick_new_cpu() and add
    lockdep_assert_cpus_held().
---
 drivers/hv/hv.c | 56 ++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 41 insertions(+), 15 deletions(-)

diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c
index 36d9ba097ff5..9fef71403c86 100644
--- a/drivers/hv/hv.c
+++ b/drivers/hv/hv.c
@@ -433,13 +433,39 @@ 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 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;
+
+	cpu = cpumask_next(get_random_u32_below(nr_cpu_ids), cpu_online_mask);
+
+	if (cpu >= nr_cpu_ids || cpu == current_cpu)
+		cpu = VMBUS_CONNECT_CPU;
+
+	ret = vmbus_channel_set_cpu(channel, 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 +482,31 @@ 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 +523,5 @@ int hv_synic_cleanup(unsigned int cpu)
 
 	hv_synic_disable_regs(cpu);
 
-	return 0;
+	return ret;
 }
-- 
2.47.1