Re: [PATCH v2] mm, hotplug: move init_currently_empty_zone() under zone_span_lock protection

[Wei Yang <richard.weiyang@gmail.com>]

On Thu, Nov 22, 2018 at 04:26:40PM +0100, David Hildenbrand wrote:
>On 22.11.18 11:12, Wei Yang wrote:
>> During online_pages phase, pgdat->nr_zones will be updated in case this
>> zone is empty.
>> 
>> Currently the online_pages phase is protected by the global lock
>> mem_hotplug_begin(), which ensures there is no contention during the
>> update of nr_zones. But this global lock introduces scalability issues.
>> 
>> This patch is a preparation for removing the global lock during
>> online_pages phase. Also this patch changes the documentation of
>> node_size_lock to include the protectioin of nr_zones.
>
>I looked into locking recently, and there is more to it.
>
>Please read:
>
>commit dee6da22efac451d361f5224a60be2796d847b51
>Author: David Hildenbrand <david@redhat.com>
>Date:   Tue Oct 30 15:10:44 2018 -0700
>
>    memory-hotplug.rst: add some details about locking internals
>    
>    Let's document the magic a bit, especially why device_hotplug_lock is
>    required when adding/removing memory and how it all play together with
>    requests to online/offline memory from user space.
>
>Short summary: Onlining/offlining of memory requires the device_hotplug_lock
>as of now.
>
>mem_hotplug_begin() is just an internal optimization. (we don't want
> everybody to take the device lock)
>

Hi, David

Thanks for your comment.

Hmm... I didn't catch your point.

Related to memory hot-plug, there are (at least) three locks,

  * device_hotplug_lock    (global)
  * device lock            (device scope)
  * mem_hotplug_lock       (global)

But with two different hold sequence in two cases:

  * device_online()

    device_hotplug_lock
    device_lock
    mem_hotplug_lock

  * add_memory_resource()

    device_hotplug_lock
    mem_hotplug_lock
    device_lock
       ^
       |
       I don't find where this is hold in add_memory_resource(). 
       Would you mind giving me a hint?

If my understanding is correct, what is your point?

I guess your point is : just remove mem_hotplug_lock is not enough to
resolve the scalability issue?

Please correct me, if I am not. :-)

-- 
Wei Yang
Help you, Help me