linux-kernel - Re: [PATCH v3] Weighted interleave auto-tuning

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Message-ID: <Z5EinHUAoHgiEL3N@gourry-fedora-PF4VCD3F>
Date: Wed, 22 Jan 2025 11:53:48 -0500
From: Gregory Price <gourry@...rry.net>
To: Joshua Hahn <joshua.hahnjy@...il.com>
Cc: "Huang, Ying" <ying.huang@...ux.alibaba.com>, hyeonggon.yoo@...com,
	rafael@...nel.org, lenb@...nel.org, gregkh@...uxfoundation.org,
	akpm@...ux-foundation.org, honggyu.kim@...com, rakie.kim@...com,
	dan.j.williams@...el.com, Jonathan.Cameron@...wei.com,
	dave.jiang@...el.com, horen.chuang@...ux.dev, hannes@...xchg.org,
	linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
	linux-mm@...ck.org, kernel-team@...a.com
Subject: Re: [PATCH v3] Weighted interleave auto-tuning

On Wed, Jan 22, 2025 at 07:59:34AM -0800, Joshua Hahn wrote:
> On Wed, 22 Jan 2025 09:37:20 +0800 "Huang, Ying" <ying.huang@...ux.alibaba.com> wrote:
> > > The current code makes sure that when you switch modes from auto
> > > to manual, it inherits the current state - instead of there being
> > > some hidden state that suddenly takes precedence.
> > 
> > I think that we can do that with two weight arrays.
> > 
> > > So I prefer to just have one IW array and no hidden state.
> > 
> > Then, when we switch from manual to auto mode, where to find
> > auto-generated weights?  Re-calculate them?
> 
> Even in manual mode, incoming bandwidth data is continuously stored.
> This way, when a user does decide to switch back to auto mode later,
> the system doesn't have to retrieve the bandwidth data all over again.
> As for the auto-generated weights, they are re-calculated based solely
> on the bandwidth data available. (I will note that re-calculating
> the weights are very quick, see reduce_interleave_weights)
> 
> Based on your description of the expected behavior, everything you
> listed out is actually what currently happens in the one-layer system.
> Switching from auto --> manual inherits the auto-generated weights, and
> switching from manual --> auto wipes all previous user-stored data.
>

Piling on - the single-layer system is very bluntly simpler (one fewer array)
with the exact same behavior.  Therefore it's better (in my opinion).

But this is all hidden from the user - so I don't care.  If you (Ying)
have strong feelings, we're happy to ship either.

~Gregory