Re: [RESEND PATCH v5 04/16] lib min_heap: Add type safe interface

[Ian Rogers]

On Wed, May 22, 2024 at 4:33 PM Kent Overstreet
<kent.overstreet@xxxxxxxxx> wrote:
>
> 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@xxxxxxxxx> 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 :)

Just to mention that the perf events use of the min heap is during a
context switch, so it is somewhat performance sensitive for people
context switching with perf events opened/enabled.

Thanks,
Ian