linux-kernel - Re: [PATCH v3 2/2] regmap: Add maple tree based register cache

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Message-ID: <6f49fdf0-c373-46f7-89bd-f30f0fb68c15@sirena.org.uk>
Date:   Mon, 3 Apr 2023 17:58:27 +0100
From:   Mark Brown <broonie@...nel.org>
To:     "Liam R. Howlett" <Liam.Howlett@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] regmap: Add maple tree based register cache

On Mon, Apr 03, 2023 at 11:45:08AM -0400, Liam R. Howlett wrote:
> * Mark Brown <broonie@...nel.org> [230329 20:10]:

> > The entries stored in the maple tree are arrays of register
> > values, with the maple tree keys holding the register addresses.

> Why not store the registers to values in the maple tree without the
> array?  From reading the below code, the maple tree will hold a ranges
> (based on registers) pointing to an array which will store the value at
> the register offset.  Could we just store the value in the maple tree
> directly?

AFAICT that means that we can't readily get the values back out en masse
to do bulk operations on them without doing a bunch of work to check for
adjacency and then doing some intermediate marshalling, with cache sync
block operations are a noticable win.  I'm *hopeful* this might end up
working out fast enough to make the cache more viable on faster buses.

> > This should work well for a lot of devices, though there's some
> > additional areas that could be looked at such as caching the
> > last accessed entry like we do for rbtree and trying to minimise
> > the maple tree level locking.

> In the case of the VMAs, we had a vmacache, which was removed when the
> maple tree was added since it wasn't providing any benefit.  We lost any
> speed increase to cache misses and updating the cache.  I don't know
> your usecase or if it would result in the same outcome here, but I
> thought I'd share what happened in the VMA space.

Yeah, I'm hopeful that the maple tree is fast enough that it's not worth
it.  The main use case is read/modify/write sequences where you hit the
same register twice in quick succession.

> > +	rcu_read_lock();
> > +
> > +	entry = mas_find(&mas, reg);

> mas_walk() might be a better interface for this.

Ah, that's not very discoverable.  mas_find() should possibly be called
mas_find_pausable() or something?

> > +	/* Any adjacent entries to extend/merge? */
> > +	mas_set_range(&mas, reg - 1, reg + 1);
> > +	index = reg;
> > +	last = reg;
> > +
> > +	lower = mas_find(&mas, reg - 1);

> If you just want to check the previous, you can use:
> mas_prev(&mas, reg - 1);
> This will try the previous entry without rewalking from the top of the
> tree and you don't need to mas_set_range() call.

Hrm, right - it looks like that doesn't actually apply the constraints
so that'd work.  The whole specifying constraints for some operations in
the mas is a bit confusing.

> > +     
> > +     mas_set_range(&mas, index, last);
> > +     ret = mas_store_gfp(&mas, entry, GFP_KERNEL);

> You can avoid this walk as well by changing the order of the code
> before:

> mas_walk(&mas, reg);
> if entry... return
> mas_next(&mas, reg + 1);
> ...
> mas_prev(&mas, reg - 1);
> ...

> This should now be pointing at the location mas_store_gfp() expects:
> mas.last = last;
> ret = mas_store_gfp()

Don't we need to set mas.index as well?  It does feel a bit wrong to be
just writing into the mas struct.

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