From: Hamza Mahfooz <hamzamahfooz@xxxxxxxxxxxxxxxxxxx> Sent: Friday, January 17, 2025 12:33 PM
>
> Currently, it is tedious to offline CPUs in a Hyper-V VM since CPUs may
> 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@xxxxxxxxx>
> Cc: Michael Kelley <mhklinux@xxxxxxxxxxx>
> Cc: Wei Liu <wei.liu@xxxxxxxxxx>
> Signed-off-by: Hamza Mahfooz <hamzamahfooz@xxxxxxxxxxxxxxxxxxx>
> ---
> 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.
>
> v5: style fixes, use target_cpu and set ret to -EBUSY by default.
> ---
> drivers/hv/hv.c | 72 ++++++++++++++++++++++++++++++++++---------------
> 1 file changed, 51 insertions(+), 21 deletions(-)
>
> diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c
> index 36d9ba097ff5..fab0690b5c41 100644
> --- a/drivers/hv/hv.c
> +++ b/drivers/hv/hv.c
> @@ -433,13 +433,47 @@ static bool hv_synic_event_pending(void)
> return pending;
> }
>
> +static int hv_pick_new_cpu(struct vmbus_channel *channel)
> +{
> + int ret = -EBUSY;
> + 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 (channel->target_cpu == cpu ||
> + channel->target_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;
> @@ -456,38 +490,34 @@ 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);
> + 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(sc);
> + 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
> - * vmbus_send_modifychannel(). Scan the event flags page looking for
> - * bits that are set and waiting with a timeout for vmbus_chan_sched()
> - * to process such bits. If bits are still set after this operation
> - * and VMBus is connected, fail the CPU offlining operation.
> + * Scan the event flags page looking for bits that are set and waiting
> + * with a timeout for vmbus_chan_sched() to process such bits. If bits
> + * are still set after this operation and VMBus is connected, fail the
> + * CPU offlining operation.
> */
> if (vmbus_proto_version >= VERSION_WIN10_V4_1 && hv_synic_event_pending())
> return -EBUSY;
> @@ -497,5 +527,5 @@ int hv_synic_cleanup(unsigned int cpu)
>
> hv_synic_disable_regs(cpu);
>
> - return 0;
> + return ret;
> }
> --
> 2.47.1
Reviewed-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
Tested-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
