On Sat, Dec 30, 2023 at 11:33:04AM +0100, Stefano Brivio wrote:
> On Thu, 28 Dec 2023 19:25:25 +0100
> Stefano Brivio <sbrivio@redhat.com> wrote:
> 
> > > On Thu, 21 Dec 2023 17:15:49 +1100
> > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > > 
> > > [...]
> >
> > [...]
> >
> > I wonder if we really have to keep track of the number of (non-)entries
> > in the free "block", and if we have to be explicit about the two cases.
> > 
> > I'm trying to find out if we can simplify the whole thing with slightly
> > different variants, for example:
> 
> So... I think the version with (explicit) blocks has this fundamental
> advantage, on deletion:
> 
> > > +	flow->f.type = FLOW_TYPE_NONE;
> > > +	/* Put it back in a length 1 free block, don't attempt to fully reverse
> > > +	 * flow_alloc()s steps.  This will get folded together the next time
> > > +	 * flow_defer_handler runs anyway() */
> > > +	flow->free.n = 1;
> > > +	flow->free.next = flow_first_free;
> > > +	flow_first_free = FLOW_IDX(flow);
> 
> which is doable even without explicit blocks, but much harder to
> follow.

Remember this is not a general deletion, only a "cancel" of the most
recent allocation.  To reduce fragmentation we are keeping the linked
list of free clusters in strictly ascending order.  The logic above is
only correct if the entry we're freeing is before any other free entry
in the table.  That will be the case for the most recent allocation,
because we always allocatte the first free entry in the table.

> Other than the comments I have, I would go ahead with your approach. My
> attempt below in case you're interested. The comment at the end of
> flow_del() shows the issue I'm facing:
> 
> struct flow_free_block {
> 	/* Must be first element */
> 	struct flow_common f;
> 
> 	unsigned next_free;
> 	unsigned next_used;
> };
> 
> static union flow *flow_alloc(void)
> {
> 	union flow *flow;
> 
> 	if (flow_first_free >= FLOW_MAX)
> 		return NULL;
> 
> 	flow = flowtab + flow_first_free;
> 
> 	flow_first_free = flow->free.next_free;
> 
> 	memset(flow, 0, sizeof(*flow));		/* TODO: select region */
> 	return flow;
> }
> 
> static void flow_del(union flow *del)
> {
> 	union flow *left, *right, *next_used = NULL, *first_free;
fg
> 
> 	del->f.type = FLOW_TYPE_NONE;
> 
> 	left =  (FLOW_IDX(del) > 0)            ? FLOW(FLOW_IDX(del) - 1) : NULL;
> 	right = (FLOW_IDX(del) < FLOW_MAX - 1) ? FLOW(FLOW_IDX(del) + 1) : NULL;
> 
> 	first_free = flow_first_free < FLOW_MAX ? FLOW(flow_first_free) : NULL;
> 
> 	if (right) {
> 		if (right->f.type == FLOW_TYPE_NONE)
> 			del->free.next_used = right->free.next_used;
> 		else
> 			del->free.next_used = right;
> 	} else {
> 		del->free.next_used = FLOW_MAX;
> 	}
> 
> 	if (left && left->f.type == FLOW_TYPE_NONE) {
> 		left->free.next_free = FLOW_IDX(del);
> 		left->free.next_used = del->free.next_used;
> 		return;
> 	}
> 
> 	if (flow_first_free == FLOW_MAX) {
> 		flow_first_free = FLOW_IDX(del);
> 	} else if (flow_first_free > FLOW_IDX(del)) {
> 		flow_first_free = FLOW_IDX(del);
> 		del->free.next_free = flow_first_free;
> 	} else if (flow_first_free < FLOW_IDX(del)) {
> 		;
> 		/* Walk free slots from flow_first_free until FLOW_IDX(del) to
> 		 * find insertion point... but that makes deletion at most O(n),
> 		 * perhaps O(log(n)), certainly not O(1).

Exactly.  I can't see any way (short of far more complex data
structures) around having a linear scan somewhere, although you can
kind of choose whether it happens on alloc or free.

The idea of my implementation is to have it at free time - but to
merge it with an existing linear scan so it won't have an additional
cost.

> 		 *
> 		 * Or just link out-of-order for the moment, and fix this up in
> 		 * flow_next(), but then "blocks" help representing this.
> 		 */
> 	}
> }
> 
> static union flow *flow_next(union flow *whence)
> {
> 	if (FLOW_IDX(whence) >= FLOW_MAX - 1)
> 		return NULL;
> 
> 	if (whence->f.type != FLOW_TYPE_NONE)
> 		whence++;
> 
> 	if (whence->f.type == FLOW_TYPE_NONE)
> 		return whence->free.next_used;
> 
> 	return whence;
> }
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson