Re: [PATCH v3 13/13] flow: Avoid moving flow entries to compact table

[Stefano Brivio <sbrivio@redhat.com>]

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.

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;

	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).
		 *
		 * 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;
}

-- 
Stefano