Re: [External Mail] [RFC PATCH] mm/mempolicy: Weighted interleave auto-tuning

[Gregory Price <gourry@gourry.net>]

On Thu, Dec 26, 2024 at 09:35:32AM +0800, Huang, Ying wrote:
> > Having two files for each node (nodeN, defaultN) seems a bit too
> > cluttered for the user perspective. Making the nodeN interfaces serve
> > multiple purposes (i.e. echo -1 into the nodes will output the default
> > value for that node) also seems a bit too complicated as well, in my
> > opinion. Maybe having a file 'weight_tables' that contains a table of
> > default/user/effective weights (as have been used in these conversations)
> > might be useful for the user? (Or maybe just the defaults)
> >
> > Then a workflow for the user may be as such:
> >
> > $ cat /sys/kernel/mm/mempolicy/weighted_interleave/weight_tables
> > default vales: [4,7,2]
> >   user values: [-,-,-]
> >     effective: [4,7,2]
> 
> AFAIK, this breaks the sysfs attribute format rule as follows.
> 
> https://docs.kernel.org/filesystems/sysfs.html#attributes
> 
> It's hard to use array sysfs attribute here too.  Because the node ID
> may be non-consecutive.  This makes it hard to read.
>

Would generally agree. I think essentially a
   use_defaults => (0 | 1)
interface is probably the best we can do.

Setting any node changes use_defaults from 1 => 0
echoing 1 into use_default clears user_values

This still allows 0 to be a manual "reset specific node to default"
mechanism for a specific node, and gives us a clean override.

The only question is a matter of hotplug behavior

nodes_online: 0,1
  default_values: [5,3]
  user_values   : [-,-]

event: node1 is taken offline
  default_values: [5,3] <-- nothing happens

event: node1 comes back online with different bandwidth attribute
  default_values: [6,5] <-- reweight as occured silently

event: user sets a custom value (node1 <= 2)
  default_values: [6,5]
  user_values:    [6,2] <= note, *no reduction*

event: node1 is taken offline
  default_values: [6,5]
  user_values:    [6,2] <= value still present but not used

event: node1 comes back online with different bandwidth attribute
  default_values: [5,3] <-- default reweight has occurred silently
  user_values   : [6,2] <-- user responsible for triggering re-weight

The user has the option of

echo 1 > /sys/.../weghted_interleave/user_defaults
result
	default_values: [5,3]
	user_values   : [-,-]
or
echo 0 > /sys/.../weighted_interleave/node1
result
	default_values: [5,3]
	user_values   : [6,3] <= only node1 is updated, no re-weight

Basically, if the user ever sets any value, we never automatically pull
new values in, and the admin is responsible for triggering a re-weight
(use_default) or manually reweighting *all* nodes - because changing
values implies a change in the bandwidth distribution anyway.

I think this makes the most sense.

~Gregory