linux-kernel - RE: [PATCH v5 3/3] drivers/hv: add CPU offlining support

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Message-ID:
 <SN6PR02MB4157B22200AECEE09849D43CD4E12@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Wed, 22 Jan 2025 15:40:05 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Hamza Mahfooz <hamzamahfooz@...ux.microsoft.com>,
	"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
CC: 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>, Thomas Gleixner <tglx@...utronix.de>, Peter
 Zijlstra <peterz@...radead.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v5 3/3] drivers/hv: add CPU offlining support

From: Hamza Mahfooz <hamzamahfooz@...ux.microsoft.com> 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@...il.com>
> Cc: Michael Kelley <mhklinux@...look.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().
> 
> 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@...look.com>
Tested-by: Michael Kelley <mhklinux@...look.com>