Re: [PATCH] Prio_heap: heap_remove(), heap_maximum(), heap_replace() and heap_cherrypick()

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* Paul Menage (menage@google.com) wrote:
> On Thu, Oct 7, 2010 at 12:03 PM, Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
> > These added interfaces lets prio_heap users lookup the top of heap item without
> > performing any insertion, perform removal of the topmost heap entry, and also
> > replacement of topmost heap entry. This is useful if one need to use the result
> > of the lookup to determine if the current maximum should simply be removed or if
> > it should be replaced.
> >
> > This is used by the Generic Ring Buffer to perform timestamp-based fusion-merge
> > of per-cpu buffer records into a single stream.
> >
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> 
> Looks fine, other than a couple of suggestions to make the code
> slightly clearer.
> 
> Acked-by: Paul Menage <menage@google.com>
> 
> > Acked-by: Balbir Singh <balbir@in.ibm.com>
> > Cc: Paul Menage <menage@google.com>
> > Cc: David Rientjes <rientjes@google.com>
> > Cc: Cedric Le Goater <clg@fr.ibm.com>
> > Cc: "Eric W. Biederman" <ebiederm@xmission.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > ---
> >  include/linux/prio_heap.h |   44 ++++++++++++++++++++++
> >  lib/prio_heap.c           |   91 ++++++++++++++++++++++++++++++++++++----------
> >  2 files changed, 116 insertions(+), 19 deletions(-)
> >
> > Index: linux.trees.git/include/linux/prio_heap.h
> > ===================================================================
> > --- linux.trees.git.orig/include/linux/prio_heap.h      2010-07-06 14:25:29.000000000 -0400
> > +++ linux.trees.git/include/linux/prio_heap.h   2010-07-07 10:04:33.000000000 -0400
> > @@ -23,6 +23,18 @@ struct ptr_heap {
> >  };
> >
> >  /**
> > + * heap_maximum - return the largest element in the heap
> > + * @heap: the heap to be operated on
> > + *
> > + * Returns the largest element in the heap, without performing any modification
> > + * to the heap structure. Returns NULL if the heap is empty.
> > + */
> > +static inline void *heap_maximum(const struct ptr_heap *heap)
> > +{
> > +       return heap->size ? heap->ptrs[0] : NULL;
> > +}
> > +
> > +/**
> >  * heap_init - initialize an empty heap with a given memory size
> >  * @heap: the heap structure to be initialized
> >  * @size: amount of memory to use in bytes
> > @@ -53,6 +65,38 @@ void heap_free(struct ptr_heap *heap);
> >  */
> >  extern void *heap_insert(struct ptr_heap *heap, void *p);
> >
> > +/**
> > + * heap_remove - remove the largest element from the heap
> > + * @heap: the heap to be operated on
> > + *
> > + * Returns the largest element in the heap. It removes this element from the
> > + * heap. Returns NULL if the heap is empty.
> > + */
> > +extern void *heap_remove(struct ptr_heap *heap);
> >
> > +/**
> > + * heap_cherrypick - remove a given element from the heap
> > + * @heap: the heap to be operated on
> > + * @p: the element
> > + *
> > + * Remove the given element from the heap. Return the element if present, else
> > + * return NULL. This algorithm has a complexity of O(n), which is higher than
> > + * O(log(n)) provided by the rest of this API.
> > + */
> > +extern void *heap_cherrypick(struct ptr_heap *heap, void *p);
> > +
> > +/**
> > + * heap_replace_max - replace the the largest element from the heap
> > + * @heap: the heap to be operated on
> > + * @p: the pointer to be inserted as topmost element replacement
> > + *
> > + * Returns the largest element in the heap. It removes this element from the
> > + * heap. The heap is rebalanced only once after the insertion. Returns NULL if
> > + * the heap is empty.
> > + *
> > + * This is the equivalent of calling heap_remove() and then heap_insert(), but
> > + * it only rebalances the heap once.
> > + */
> > +extern void *heap_replace_max(struct ptr_heap *heap, void *p);
> >
> >  #endif /* _LINUX_PRIO_HEAP_H */
> > Index: linux.trees.git/lib/prio_heap.c
> > ===================================================================
> > --- linux.trees.git.orig/lib/prio_heap.c        2010-07-06 14:25:29.000000000 -0400
> > +++ linux.trees.git/lib/prio_heap.c     2010-07-07 10:18:32.000000000 -0400
> > @@ -23,12 +23,49 @@ void heap_free(struct ptr_heap *heap)
> >        kfree(heap->ptrs);
> >  }
> >
> > -void *heap_insert(struct ptr_heap *heap, void *p)
> > +static void heapify(struct ptr_heap *heap, void **ptrs, void *p, int pos)
> 
> I'd be inclined to not pass "ptrs" or "p" as an argument, and instead
> get ptrs from heap->ptrs and p from ptrs[pos]. It makes it clearer
> what heapify() is doing.

OK.

> 
> > +void *heap_replace_max(struct ptr_heap *heap, void *p)
> >  {
> >        void *res;
> >        void **ptrs = heap->ptrs;
> >        int pos;
> 
> No need for "pos" - just pass a constant 0 to heapify()

OK

> >
> > +       if (!heap->size) {
> > +               ptrs[heap->size++] = p;
> 
> Maybe make this clearer?
> 
>   ptrs[0] = p;
>   heap->size = 1;

OK.

> 
> > +void *heap_remove(struct ptr_heap *heap)
> > +{
> > +       void **ptrs = heap->ptrs;
> > +
> > +       switch (heap->size) {
> > +       case 0:
> > +               return NULL;
> > +       case 1:
> > +               return ptrs[--heap->size];
> 
> Maybe clearer:
> 
> heap->size = 0;
> return ptrs[0];

Yep, I changed the same thing in heap_cherrypick too.

Thanks!

Mathieu


-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com