linux-kernel - Re: [PATCH 1/2] augmented rbtree: use max3() in the *_compute

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Date:   Tue, 13 Aug 2019 11:00:42 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Michel Lespinasse <walken@...gle.com>
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Roman Gushchin <guro@...com>, Hillf Danton <hdanton@...a.com>,
        Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 1/2] augmented rbtree: use max3() in the *_compute_max()
 function

> On Sun, Aug 11, 2019 at 11:46 AM Uladzislau Rezki (Sony)
> <urezki@...il.com> wrote:
> >
> > Recently there was introduced RB_DECLARE_CALLBACKS_MAX template.
> > One of the callback, to be more specific *_compute_max(), calculates
> > a maximum scalar value of node against its left/right sub-tree.
> >
> > To simplify the code and improve readability we can switch and
> > make use of max3() macro that makes the code more transparent.
> >
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> 
> Thanks. The change is correct but I think I prefer it the "before"
> version. My reasons are:
> 
> - I don't have a strong style preference either way - it's the same
> amount of code either way, admittedly more modular in your proposal,
> but also with more indirection (compute_max refers to get_max and
> max3). The indirection doesn't hinder readability but IMO it makes it
> harder to be confident that the compiler will generate quality code,
> compared to the "before" approach which just lays down all the pieces
> in a linear way.
Thank you for your comments. As for compiler and what can be generated
as a result depends on arch, etc, so i agree here. "inline" is a hint only.
But it can be rewritten. One way is to use __always_inline another one is:

<snip>
RBTYPE max = max3(RBCOMPUTE(node),				    \
	node->RBFIELD.rb_left ?					    \
		rb_entry(node->RBFIELD.rb_left,			    \
			RBSTRUCT, RBFIELD)->RBAUGMENTED:0,	    \
	node->RBFIELD.rb_right ?				    \
		rb_entry(node->RBFIELD.rb_right,		    \
			RBSTRUCT, RBFIELD)->RBAUGMENTED:0);
<snip>

i.e. directly embed an access to the left/right nodes into max3().
That way we can get rid of extra "child" variable and to have a liner
code as "before" variant.

Again, i am not interested in just pushing this change, the aim was
to make it more readable for others and that is it.  

> 
> - A quick check shows that the proposed change generates larger code
> for mm/interval_tree.o:
>    2757       0       0    2757     ac5 mm/interval_tree.o
>    2533       0       0    2533     9e5 mm/interval_tree.o.orig
>   This does not happen for every RB_DECLARE_CALLBACKS_MAX use,
> lib/interval_tree.o in particular seems to be fine. But it does go
> towards my gut feeling that the change trusts the compiler/optimizer
> more than I want to.
> 
I see your point. Indeed the generated code is bit bigger with the change,
however with above modification it improves the situation and becomes:
<snip>
284544 Aug 13 09:53 interval_tree.o
283192 Aug 13 09:57 interval_tree.o.orig
<snip>

but is still a bit higher. If we care about that, then i will drop this patch,
because "before" code is better in that context.

> - Slight loss of generality. The "before" code only assumes that the
> RBAUGMENTED field can be compared using "<" ; the "after" code also
> assumes that the minimum value is 0. While this covers the current
> uses, I would prefer not to have that limitation.
If we care about negative augmented values, then we should stick to
"before" code. Agree here. If you have any ideas how to extend and
cover negative cases, please let me know. Otherwise we can drop this
change and do not pay much attention at it.

Thank you.

--
Vlad Rezki