linux-kernel - Re: [RESEND PATCH v5 04/16] lib min

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Message-ID: <gch7c2merdegxvqwpbb5ean7uj35tnh62fyviggjglftlfjs3i@jhbnllco5kau>
Date: Wed, 22 May 2024 19:33:04 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Kuan-Wei Chiu <visitorckw@...il.com>, colyli@...e.de, 
	msakai@...hat.com, peterz@...radead.org, mingo@...hat.com, acme@...nel.org, 
	namhyung@...nel.org, bfoster@...hat.com, mark.rutland@....com, 
	alexander.shishkin@...ux.intel.com, jolsa@...nel.org, irogers@...gle.com, adrian.hunter@...el.com, 
	bagasdotme@...il.com, jserv@...s.ncku.edu.tw, linux-bcache@...r.kernel.org, 
	linux-kernel@...r.kernel.org, dm-devel@...ts.linux.dev, linux-bcachefs@...r.kernel.org, 
	linux-perf-users@...r.kernel.org
Subject: Re: [RESEND PATCH v5 04/16] lib min_heap: Add type safe interface

On Wed, May 22, 2024 at 04:10:48PM -0700, Andrew Morton wrote:
> On Tue, 14 May 2024 16:47:12 +0800 Kuan-Wei Chiu <visitorckw@...il.com> wrote:
> 
> > Implement a type-safe interface for min_heap using strong type
> > pointers instead of void * in the data field. This change includes
> > adding small macro wrappers around functions, enabling the use of
> > __minheap_cast and __minheap_obj_size macros for type casting and
> > obtaining element size. This implementation removes the necessity of
> > passing element size in min_heap_callbacks. Additionally, introduce the
> > MIN_HEAP_PREALLOCATED macro for preallocating some elements.
> 
> I guess it's a nice change, although it's unclear how valuable all this
> is.  Being able to remove the local implementations in bcache and
> bcachefs is good.

Yes, and now the standard kernel version is type safe. I suggested it to
Kuan as a good starting project, I find (well picked) refactorings and
cleanups are a great place for people to start.

> 
> > --- a/drivers/md/dm-vdo/repair.c
> > +++ b/drivers/md/dm-vdo/repair.c
> > @@ -51,6 +51,8 @@ struct recovery_point {
> >  	bool increment_applied;
> >  };
> >  
> > +MIN_HEAP(struct numbered_block_mapping, replay_heap);
> >
> > ...
> >
> > -struct min_heap {
> > -	void *data;
> > -	int nr;
> > -	int size;
> > -};
> > +#define MIN_HEAP_PREALLOCATED(_type, _name, _nr)	\
> > +struct _name {	\
> > +	int nr;	\
> > +	int size;	\
> > +	_type *data;	\
> > +	_type preallocated[_nr];	\
> > +}
> 
> I think that the MIN_HEAP() macro would be better named
> DEFINE_MIN_HEAP().  There's a lot of precedent for this and it's
> unclear whether "MIN_HEAP" actually implements storage.  I looked at
> the repair.c addition and thought "hm, shouldn't that be static"!

Agreed

> >  /* Sift the element at pos down the heap. */
> >  static __always_inline
> > -void min_heapify(struct min_heap *heap, int pos,
> > +void __min_heapify(min_heap_char *heap, int pos, size_t elem_size,
> >  		const struct min_heap_callbacks *func)
> >  {
> 
> It's a separate project, but min_heap.h has some crazily huge inlined
> functions.  I expect we'd have a smaller and faster kernel if those
> were moved into a new lib/min_heap.c.

good thought - Kuan, want to do that too? Since we missed the merge
window, may as well :)