Re: [PATCH v2 3/3] powerpc/mm/hash: Avoid multiple HPT resize-downs on memory hotunplug

["Leonardo Brás" <leobras.c@gmail.com>]

On Mon, 2021-06-07 at 15:20 +1000, David Gibson wrote:
> On Fri, Apr 30, 2021 at 11:36:10AM -0300, Leonardo Bras wrote:
> > During memory hotunplug, after each LMB is removed, the HPT may be
> > resized-down if it would map a max of 4 times the current amount of
> > memory.
> > (2 shifts, due to introduced histeresis)
> > 
> > It usually is not an issue, but it can take a lot of time if HPT
> > resizing-down fails. This happens  because resize-down failures
> > usually repeat at each LMB removal, until there are no more bolted
> > entries
> > conflict, which can take a while to happen.
> > 
> > This can be solved by doing a single HPT resize at the end of
> > memory
> > hotunplug, after all requested entries are removed.
> > 
> > To make this happen, it's necessary to temporarily disable all HPT
> > resize-downs before hotunplug, re-enable them after hotunplug ends,
> > and then resize-down HPT to the current memory size.
> > 
> > As an example, hotunplugging 256GB from a 385GB guest took 621s
> > without
> > this patch, and 100s after applied.
> > 
> > Signed-off-by: Leonardo Bras <leobras.c@gmail.com>
> 
> Hrm.  This looks correct, but it seems overly complicated.
> 
> AFAICT, the resize calls that this adds should in practice be the
> *only* times we call resize, all the calls from the lower level code
> should be suppressed. 

That's correct.

>  In which case can't we just remove those calls
> entirely, and not deal with the clunky locking and exclusion here.
> That should also remove the need for the 'shrinking' parameter in
> 1/3.


If I get your suggestion correctly, you suggest something like:
1 - Never calling resize_hpt_for_hotplug() in
hash__remove_section_mapping(), thus not needing the srinking
parameter.
2 - Functions in hotplug-memory.c that call dlpar_remove_lmb() would in
fact call another function to do the batch resize_hpt_for_hotplug() for
them

If so, that assumes that no other function that currently calls
resize_hpt_for_hotplug() under another path, or if they do, it does not
need to actually resize the HPT.

Is the above correct?

There are some examples of functions that currently call
resize_hpt_for_hotplug() by another path:

add_memory_driver_managed
	virtio_mem_add_memory
	dev_dax_kmem_probe

reserve_additional_memory
	balloon_process
	add_ballooned_pages

__add_memory
	probe_store

__remove_memory
	pseries_remove_memblock

remove_memory
	dev_dax_kmem_remove
	virtio_mem_remove_memory

memunmap_pages
	pci_p2pdma_add_resource
	virtio_fs_setup_dax


Best regards,
Leonardo Bras