linux-kernel - Re: [PATCH v2] maple_tree: simplify mas_wr_node

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Message-ID: <k7mse2jyysazfayjayamr75dxtsjx3dbgvci65jhaaiye2l7qi@hwwvinj2mgoa>
Date: Fri, 23 Aug 2024 13:51:28 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Hao Li <haoli.tcs@...il.com>
Cc: Peng Zhang <zhangpeng.00@...edance.com>, akpm@...ux-foundation.org,
        maple-tree@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] maple_tree: simplify mas_wr_node_walk for improved
 readability

* Hao Li <haoli.tcs@...il.com> [240823 05:38]:
> On Fri, Aug 23, 2024 at 05:07:31PM +0800, Peng Zhang wrote:
> > 
> > 
> > 在 2024/8/23 16:17, Hao Li 写道:
> > > Refactor mas_wr_node_walk to make the code more clear and easier to
> > > understand. The main changes are:

Thank you for your patch, but I don't think this is a good change.

NACK

> > > 
> > > 1. Replace the forward-iterating loop with a backward-iterating loop.
> > >   This simplifies the logic for determining the correct range
> > >   containing mas->index.
> > I don't think iterating in reverse is a good idea, it makes the code
> > different from everywhere else.
> 

I also agree with Peng that having one loop go a different way seems
like it's asking for trouble.

> I understand your concern about consistency in iteration direction.
> 
> While the original code correctly handled all cases, the process wasn't
> as definitive or clear.
> 
> The new approach unifies the logic by eliminating the need to treat
> `offset >= count` as a special case. This results in a more
> straightforward and consistent flow throughout the function, which
> provides a more deterministic and easy-to-follow path through the logic.
> We can more clearly see and understand how we're determining the correct
> range for `mas->index` without having to mentally parse complex
> conditional logic.
> 
> > > 
> > > 2. Eliminate the ternary operator.

This is not more clear as one may miss that the loop may not execute at
all. So you eliminated the ternary operator, but have all but hidden the
assignment.  I would rather an if/else for verbosity, but not enough to
reject the patch that added it in the first place.

You also replaced the "unsigned char count" with an "int idx", for some
reason.  It seems like you've rewritten it so it's more clear for you.

> > > 
> > > The new implementation maintains the same functionality as before, but
> > > with improved readability. The performance characteristics remain
> > > essentially the same, as we cannot predict which interval mas->index
> > > will fall into.

We do favour the left side of the tree to increase data density, so it
is more likely to find what we are looking for in the lower slots (but
not by a whole lot right now). There will probably be more of this
favouring in the future - minimum span for internal nodes.

BENCH_NODE_STORE went from 8.79, 8.85, 8.79 seconds to 9.68, 9.74, 9.72.
This change is slower.  It may be because you removed all the temporary
variables that avoided dereferencing, so the compiler can't be as smart
about optimisation.  I'm not really interested in finding out why it's
slower as I don't think this is a good change on the grounds of other
reasons stated as well.

Thanks,
Liam

> > > 
> > > Signed-off-by: Hao Li <haoli.tcs@...il.com>
> > > ---
> > >   lib/maple_tree.c | 18 ++++++++----------
> > >   1 file changed, 8 insertions(+), 10 deletions(-)
> > > 
> > > diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> > > index fe1b01b29..0b3eb55d8 100644
> > > --- a/lib/maple_tree.c
> > > +++ b/lib/maple_tree.c
> > > @@ -2203,7 +2203,7 @@ static inline void mas_node_or_none(struct ma_state *mas,
> > >   static inline void mas_wr_node_walk(struct ma_wr_state *wr_mas)
> > >   {
> > >   	struct ma_state *mas = wr_mas->mas;
> > > -	unsigned char count, offset;
> > > +	int idx;
> > >   	if (unlikely(ma_is_dense(wr_mas->type))) {
> > >   		wr_mas->r_max = wr_mas->r_min = mas->index;
> > > @@ -2213,16 +2213,14 @@ static inline void mas_wr_node_walk(struct ma_wr_state *wr_mas)
> > >   	wr_mas->node = mas_mn(wr_mas->mas);
> > >   	wr_mas->pivots = ma_pivots(wr_mas->node, wr_mas->type);
> > > -	count = mas->end = ma_data_end(wr_mas->node, wr_mas->type,
> > > +	mas->end = ma_data_end(wr_mas->node, wr_mas->type,
> > >   				       wr_mas->pivots, mas->max);
> > > -	offset = mas->offset;
> > > -
> > > -	while (offset < count && mas->index > wr_mas->pivots[offset])
> > > -		offset++;
> > > -
> > > -	wr_mas->r_max = offset < count ? wr_mas->pivots[offset] : mas->max;
> > > -	wr_mas->r_min = mas_safe_min(mas, wr_mas->pivots, offset);
> > > -	wr_mas->offset_end = mas->offset = offset;
> > > +	wr_mas->r_max = mas->max;
> > > +	idx = mas->end - 1;
> > > +	while (idx >= mas->offset && wr_mas->pivots[idx] >= mas->index)
> > > +		wr_mas->r_max = wr_mas->pivots[idx--];
> > > +	wr_mas->offset_end = mas->offset = idx + 1;
> > > +	wr_mas->r_min = mas_safe_min(mas, wr_mas->pivots, mas->offset);
> > >   }
> > >   /*