Re: [RFC PATCH 1/2] Drivers: hv: vmbus: Re-balance channel interrupts across CPUs at CPU hotplug

Andrea Parri
2020-06-06T21:49:22Z

> > @@ -515,6 +519,8 @@ static void vmbus_add_channel_work(struct work_struct *work)
> > if (ret != 0) {
> > pr_err("unable to add child device object (relid %d)\n",
> > newchannel->offermsg.child_relid);
> > + if (hv_is_perf_channel(newchannel))
> > + free_chn_counts(&newchannel->device_obj->chn_cnt);
>
> You could drop the "if" condition and just always call free_chn_counts() since
> it will do the right thing for non-perf channels where the memory was never
> allocated.

Well, AFAICT, the above would do the "right" thing for non-perf channels
without calling kfree() twice. ;-) It would also serve as a hard-coded
"reminder" of the fact that there is no alloc_chn_counts() for them. No
strong opinions though, will drop for the next submission...

> > +static void filter_vp_index(struct hv_device *hv_dev, cpumask_var_t cpu_msk)
> > +{
> > + /*
> > + * The selection of the target CPUs is performed in two domains,
> > + * the device domain and the connection domain. At each domain,
> > + * in turn, invalid CPUs are filtered out at two levels, the CPU
>
> I would drop the word "invalid". You are filtering out CPUs that meet the
> criteria in the sentence starting after the colon below.

Agreed, will drop.

> > +static void balance_vp_index(struct vmbus_channel *chn,
> > + struct hv_device *hv_dev, cpumask_var_t cpu_msk)
> > +{
> > + u32 cur_cpu = chn->target_cpu, tgt_cpu = cur_cpu;
> > +
> > + if (chn->state != CHANNEL_OPENED_STATE) {
> > + /*
> > + * The channel may never have been opened or it may be in
> > + * a closed/closing state; if so, do not touch the target
> > + * CPU of this channel.
> > + */
> > + goto update_chn_cnts;
> > + }
> > +
> > + /*
> > + * The channel was open, and it will remain open until we release
> > + * channel_mutex, cf. the use of channel_mutex and channel->state
> > + * in vmbus_disconnect_ring() -> vmbus_close_internal().
> > + */
> > +
> > + if (!hv_is_perf_channel(chn)) {
> > + /*
> > + * Only used by the CPU hot removal path, remark that
> > + * the connect CPU can not go offline.
>
> To be super explicit in the comment: If the channel is not a
> performance channel, then it does not participate in the balancing,
> and we move it back to interrupting the VMBUS_CONNECT_CPU for
> lack of a better choice. Because non-perf channels are initially set to
> interrupt the VMBUS_CONNECT_CPU, the only way a non-perf channel
> could be found in this state (i.e., set to a CPU other than
> VMBUS_CONNECT_CPU) is a manual change through the sysfs interface.

The comment was indeed rather meant to make explicit a "please go read
the caller, carefully..." where, among other things, at least parts of
the remarks you pointed out above are spelled out. But I won't be the
one stingy with comments! ;-) Will apply, thanks for the suggestion.

> > +void vmbus_balance_vp_indexes_at_cpuhp(unsigned int cpu, bool add)
> > +{
> > + struct vmbus_channel *chn, *sc;
> > + cpumask_var_t cpu_msk;
> > +
> > + lockdep_assert_cpus_held();
> > + lockdep_assert_held(&vmbus_connection.channel_mutex);
> > +
> > + WARN_ON(!cpumask_test_cpu(cpu, cpu_online_mask));
> > +
> > + /* See the header comment for vmbus_send_modifychannel(). */
> > + if (vmbus_proto_version < VERSION_WIN10_V4_1)
> > + return;
> > +
> > + if (!alloc_cpumask_var(&cpu_msk, GFP_KERNEL))
> > + return;
> > +
> > + reset_chn_counts(&vmbus_connection.chn_cnt);
> > +
> > + list_for_each_entry(chn, &vmbus_connection.chn_list, listentry) {
> > + struct hv_device *dev = chn->device_obj;
> > +
> > + /*
> > + * The device may not have been allocated/assigned to
> > + * the primary channel yet; if so, do not balance the
> > + * channels associated to this device. If dev != NULL,
> > + * the synchronization on channel_mutex ensures that
> > + * the device's chn_cnt has been (properly) allocated
> > + * *and* initialized, cf. vmbus_add_channel_work().
> > + */
> > + if (dev == NULL)
> > + continue;
> > +
> > + /*
> > + * By design, non-"perf" channels do not take part in
> > + * the balancing process.
> > + *
> > + * The user may have assigned some non-"perf" channel
> > + * to this CPU. To satisfy the user's request to hot
> > + * remove the CPU, we will re-assign such channels to
> > + * the connect CPU; cf. balance_vp_index().
> > + */
> > + if (!hv_is_perf_channel(chn)) {
> > + if (add)
> > + continue;
> > + /*
> > + * Assume that the non-"perf" channel has no
> > + * sub-channels.
> > + */
>
> The "if" statement below could use a bit further explanation to help
> the reader. :-) If this non-perf channel is assigned to some CPU other
> than the one we are hot-removing, then we execute the "continue"
> statement and leave its target CPU unchanged. But if it is assigned
> to the CPU we are hot removing, then we need to move the channel
> to some other CPU.
>
> I'm also not clear on how the above comment about having no
> sub-channels is relevant. Maybe a bit more explanation would
> help.

That comment was meant to simply stress the fact that we can "continue"
without looping over/checking the sub-channels, because we know that the
channel in question has no sub-channels. ;-) (BTW, for the very same
reason, we have no "if (!hv_is_perf_channel(sc))..." in the loop below.)

So, maybe something like:

/*
* If this non-"perf" channel is assigned to some...
* [your text/explanation above].
*/
if (chn->target_cpu != cpu) {
/*
* Non-"perf" channels have no sub-channels:
* no need to loop over sc_list.
*/
continue;
}

??

> > + if (chn->target_cpu != cpu)
> > + continue;
> > + } else {
> > + reset_chn_counts(&dev->chn_cnt);
> > + }
> > +
> > + cpumask_copy(cpu_msk, cpu_online_mask);
> > + if (!add)
> > + cpumask_clear_cpu(cpu, cpu_msk);
> > + balance_vp_index(chn, dev, cpu_msk);
> > +
> > + list_for_each_entry(sc, &chn->sc_list, sc_list) {
> > + cpumask_copy(cpu_msk, cpu_online_mask);
> > + if (!add)
> > + cpumask_clear_cpu(cpu, cpu_msk);
> > + balance_vp_index(sc, dev, cpu_msk);
> > + }
> > + }
> > +
> > + free_cpumask_var(cpu_msk);
> > +}

> > int hv_synic_init(unsigned int cpu)
> > {
> > + /*
> > + * The CPU has been hot added: try to re-balance the channels
> > + * across the online CPUs (including "this" CPU), provided that
> > + * the VMBus is connected; in part., avoid the re-balancing at
> > + * the very first CPU initialization.
> > + *
> > + * See also inline comments in hv_synic_cleanup().
> > + */
> > + if (vmbus_connection.conn_state == CONNECTED) {
> > + mutex_lock(&vmbus_connection.channel_mutex);
> > + vmbus_balance_vp_indexes_at_cpuhp(cpu, true);
>
> Does the target CPU have its bit in cpu_online_mask set at the time this
> is executed? reset_chn_counts() does a for_each_online_cpu() loop, and
> we want to make sure the count for this CPU gets reset to zero.

Yes, it does: We're here (in CPUHP_AP_ONLINE_DYN) near the end of the
"CPU-online" process; IIUC, the bit in question is set earlier in this
process/before the CPU reaches CPUHP_AP_ONLINE_IDLE.

So, yeah, I think I would agree in saying that that:

WARN_ON(!cpumask_test_cpu(cpu, cpu_online_mask));

(in vmbus_balance_vp_indexes_at_cpuhp()) is more about "paranoid" (for
future changes/uses) than anything else. I'm keeping that for now.

> > @@ -980,6 +980,9 @@ static void vmbus_device_release(struct device *device)
> > mutex_lock(&vmbus_connection.channel_mutex);
> > hv_process_channel_removal(channel);
> > mutex_unlock(&vmbus_connection.channel_mutex);
> > +
> > + if (hv_is_perf_channel(channel))
> > + free_chn_counts(&hv_dev->chn_cnt);
>
> Again, can drop the 'if' statement.

As mentioned above, either way works for me. Will drop it for the next
version.

Thanks,
Andrea