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

[Gregory Price]

On Sat, Dec 21, 2024 at 01:57:58PM +0800, Huang, Ying wrote:
> Hi, Joshua,
> 
> Joshua Hahn <joshua.hahnjy@xxxxxxxxx> writes:
> 
> > On Fri, 13 Dec 2024 15:19:20 +0900 Hyeonggon Yoo <hyeonggon.yoo@xxxxxx> wrote:
> >
> >> On 2024-12-11 06:54 AM, Joshua Hahn wrote:
> 
> [snip]
> 
> >
> > [-----8<-----]
> >
> >> > +What:		/sys/kernel/mm/mempolicy/weighted_interleave/max_node_weight
> 
> I don't think that we need a new knob for this.  Just use a reasonable
> default value, for example, 32 or 16.  If it turns out that a knob will
> be really helpful, we can add it at that time.  For now, I don't think
> the requirements are clear.  And, this is a new ABI and we need to
> maintain it almost for ever.  We must be careful about new knob.
> 

This is fair.  We spent a good amount of time modeling the best
effective maximum weight and basically came to the conclusion that 32
has a good balance of minimizing error and being somewhat aggressive.

Ripping out the sysfs is easy enough.

> >
> > Regardless of what implementation makes sense, I can re-write the
> > description so that there is no ambiguity when it comes to the
> > expected behavior of the code. Thank you for pointing this out!
> 
> I don't think that it's a good idea to override the user supplied
> configuration values.  User configurations always have higher priority
> than system default configurations.  IIUC, this is the general rule of
> Linux kernel user space interface.
> 

We discussed this and decided it was confusing no matter what we did.

If new data comes in (CDAT data from a hotplug event), then the weights
are now wrong for the new global state - regardless of whether the user
set a weight manually or not.  This also allowed us to simplify the
implementation a bit.

But if generally we need to preserve user settings, then I think the
best we can do to provide a sane system is ignore the user setting when
re-weighting on a hotplug event.

e.g. user has not set a value

default_values [5,2,-] <- 1 node not set, expected to be hotplugged
user_values    [-,-,-] <- user has not set values
effective      [5,2,-]

hotplug event
default_values [2,1,1] - reweight has occurred
user_values    [-,-,-]
effective      [2,1,1]

e.g. user has set a value

default_values [5,2,-] <- 1 node not set, expected to be hotplugged
user_values    [4,-,-] <- user has only set one value
effective      [4,2,-]

hotplug event
default_values [2,1,1] - reweight has occurred
user_values    [4,-,-]
effective      [4,1,1]


So default values get updated, but user values get left alone.

If that's sane we'll fix it up.

> ---
> Best Regards,
> Huang, Ying