Hi Hamza,
Thanks for the patch, a few comments below:
On Fri, Jan 10, 2025 at 03:05:06PM -0500, Hamza Mahfooz wrote:
> 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@xxxxxxxxx>
> Cc: Wei Liu <wei.liu@xxxxxxxxxx>
> Signed-off-by: Hamza Mahfooz <hamzamahfooz@xxxxxxxxxxxxxxxxxxx>
> ---
> drivers/hv/hv.c | 57 +++++++++++++++++++++++++++++++-----------
> drivers/hv/vmbus_drv.c | 51 +++++++++++++++++++++----------------
> include/linux/hyperv.h | 1 +
> 3 files changed, 73 insertions(+), 36 deletions(-)
>
> diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c
> index 36d9ba097ff5..42270a7a7a19 100644
> --- a/drivers/hv/hv.c
> +++ b/drivers/hv/hv.c
> @@ -433,13 +433,40 @@ 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_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;
> +
> + cpus_read_lock();
hv_pick_new_cpu() is only called inside hv_synic_cleanup(), which is
only called with cpus_read_lock() held (because it's registered via
cpuhp_setup_state_nocalls_cpuslocked()). So the cpus_read_lock() is not
necessary here if I'm not missing anything. Moreover, given
cpus_read_lock() is a non-recursive read lock, it's actually incorrect
to re-acquire it here, see:
https://docs.kernel.org/locking/lockdep-design.html#recursive-read-locks
for more information.
> + 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);
> + cpus_read_unlock();
> +
> + 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 +483,31 @@ 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 +524,5 @@ int hv_synic_cleanup(unsigned int cpu)
>
> hv_synic_disable_regs(cpu);
>
> - return 0;
> + return ret;
> }
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index 2892b8da20a5..c256e02fa66b 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -1611,16 +1611,15 @@ static ssize_t target_cpu_show(struct vmbus_channel *channel, char *buf)
> {
> return sprintf(buf, "%u\n", channel->target_cpu);
> }
> -static ssize_t target_cpu_store(struct vmbus_channel *channel,
It'll be nice if you could separate this change into a different patch,
that is one patch for refactoring target_cpu_store() with
vmbus_channel_set_cpu() and the other patch to introduce the support of
cpu offline. Make it easier for reviewing.
> - const char *buf, size_t count)
> +
> +int vmbus_channel_set_cpu(struct vmbus_channel *channel, u32 target_cpu)
> {
> - u32 target_cpu, origin_cpu;
> - ssize_t ret = count;
> + u32 origin_cpu;
> + int ret = 0;
>
> - if (vmbus_proto_version < VERSION_WIN10_V4_1)
> - return -EIO;
> + lockdep_assert_held(&vmbus_connection.channel_mutex);
Add
lockdep_assert_cpus_held();
as well?
Regards,
Boqun
>
> - if (sscanf(buf, "%uu", &target_cpu) != 1)
> + if (vmbus_proto_version < VERSION_WIN10_V4_1)
> return -EIO;
>
> /* Validate target_cpu for the cpumask_test_cpu() operation below. */
> @@ -1630,22 +1629,17 @@ static ssize_t target_cpu_store(struct vmbus_channel *channel,
> if (!cpumask_test_cpu(target_cpu, housekeeping_cpumask(HK_TYPE_MANAGED_IRQ)))
> return -EINVAL;
>
> - /* No CPUs should come up or down during this. */
> - cpus_read_lock();
> -
> - if (!cpu_online(target_cpu)) {
> - cpus_read_unlock();
> + if (!cpu_online(target_cpu))
> return -EINVAL;
> - }
>
> /*
> - * Synchronizes target_cpu_store() and channel closure:
> + * Synchronizes vmbus_channel_set_cpu() and channel closure:
> *
> * { Initially: state = CHANNEL_OPENED }
> *
> * CPU1 CPU2
> *
> - * [target_cpu_store()] [vmbus_disconnect_ring()]
> + * [vmbus_channel_set_cpu()] [vmbus_disconnect_ring()]
> *
> * LOCK channel_mutex LOCK channel_mutex
> * LOAD r1 = state LOAD r2 = state
> @@ -1660,7 +1654,6 @@ static ssize_t target_cpu_store(struct vmbus_channel *channel,
> * Note. The host processes the channel messages "sequentially", in
> * the order in which they are received on a per-partition basis.
> */
> - mutex_lock(&vmbus_connection.channel_mutex);
>
> /*
> * Hyper-V will ignore MODIFYCHANNEL messages for "non-open" channels;
> @@ -1668,17 +1661,17 @@ static ssize_t target_cpu_store(struct vmbus_channel *channel,
> */
> if (channel->state != CHANNEL_OPENED_STATE) {
> ret = -EIO;
> - goto cpu_store_unlock;
> + goto end;
> }
>
> origin_cpu = channel->target_cpu;
> if (target_cpu == origin_cpu)
> - goto cpu_store_unlock;
> + goto end;
>
> if (vmbus_send_modifychannel(channel,
> hv_cpu_number_to_vp_number(target_cpu))) {
> ret = -EIO;
> - goto cpu_store_unlock;
> + goto end;
> }
>
> /*
> @@ -1708,9 +1701,25 @@ static ssize_t target_cpu_store(struct vmbus_channel *channel,
> origin_cpu, target_cpu);
> }
>
> -cpu_store_unlock:
> - mutex_unlock(&vmbus_connection.channel_mutex);
> +end:
> + return ret;
> +}
> +
> +static ssize_t target_cpu_store(struct vmbus_channel *channel,
> + const char *buf, size_t count)
> +{
> + ssize_t ret = count;
> + u32 target_cpu;
> +
> + if (sscanf(buf, "%uu", &target_cpu) != 1)
> + return -EIO;
> +
> + mutex_lock(&vmbus_connection.channel_mutex);
> + cpus_read_lock();
> + ret = vmbus_channel_set_cpu(channel, target_cpu);
> cpus_read_unlock();
> + mutex_unlock(&vmbus_connection.channel_mutex);
> +
> return ret;
> }
> static VMBUS_CHAN_ATTR(cpu, 0644, target_cpu_show, target_cpu_store);
> diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
> index 02a226bcf0ed..25e9e982f1b0 100644
> --- a/include/linux/hyperv.h
> +++ b/include/linux/hyperv.h
> @@ -1670,6 +1670,7 @@ int vmbus_send_tl_connect_request(const guid_t *shv_guest_servie_id,
> const guid_t *shv_host_servie_id);
> int vmbus_send_modifychannel(struct vmbus_channel *channel, u32 target_vp);
> void vmbus_set_event(struct vmbus_channel *channel);
> +int vmbus_channel_set_cpu(struct vmbus_channel *channel, u32 target_cpu);
>
> /* Get the start of the ring buffer. */
> static inline void *
> --
> 2.47.1
>
