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

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Message-ID: <Z2bXgMw5WPCqwGSk@gourry-fedora-PF4VCD3F>
Date: Sat, 21 Dec 2024 09:58:08 -0500
From: Gregory Price <gourry@...rry.net>
To: "Huang, Ying" <ying.huang@...ux.alibaba.com>
Cc: Joshua Hahn <joshua.hahnjy@...il.com>, hyeonggon.yoo@...com,
	"gourry@...rry.net" <gourry@...rry.net>, kernel_team@...ynix.com,
	"rafael@...nel.org" <rafael@...nel.org>,
	"lenb@...nel.org" <lenb@...nel.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	김홍규(KIM HONGGYU) System SW <honggyu.kim@...com>,
	김락기(KIM RAKIE) System SW <rakie.kim@...com>,
	"dan.j.williams@...el.com" <dan.j.williams@...el.com>,
	"Jonathan.Cameron@...wei.com" <Jonathan.Cameron@...wei.com>,
	"dave.jiang@...el.com" <dave.jiang@...el.com>,
	"horen.chuang@...ux.dev" <horen.chuang@...ux.dev>,
	"hannes@...xchg.org" <hannes@...xchg.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"kernel-team@...a.com" <kernel-team@...a.com>
Subject: Re: [External Mail] [RFC PATCH] mm/mempolicy: Weighted interleave
 auto-tuning

On Sat, Dec 21, 2024 at 01:57:58PM +0800, Huang, Ying wrote:
> Hi, Joshua,
> 
> Joshua Hahn <joshua.hahnjy@...il.com> writes:
> 
> > On Fri, 13 Dec 2024 15:19:20 +0900 Hyeonggon Yoo <hyeonggon.yoo@...com> 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