Re: [PATCH 2/2] maple_tree: add a fast path case in mas_wr_slot_store()

[Liam R. Howlett]

* Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx> [230605 07:11]:
> 
> 
> 在 2023/6/3 00:41, Liam R. Howlett 写道:
> > * Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx> [230602 03:54]:
> > > When the new range overwrites three ranges and does not touch the
> > > boundaries on both sides, the number of entries will not be increased,
> > > so we can just update the pivots as a fast path. However, it may
> > > introduce potential risks in RCU mode (although it can pass the test),
> > > because it updates two pivots. We only enable it in non-RCU mode for now.
> > 
> > So what you are saying is that you are expanding one entry to consume
> > portions of the previous and next into a new entry.  We know this is the
> > case because the end of the node is not moving and we are modifying more
> > than one slot (so it must be 2 slots)
> > 
> > This scenario is not tested in the testing framework.  We should add
> > testing before we can add this.
> > 
> > > 
> > > Signed-off-by: Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx>
> > > ---
> > >   lib/maple_tree.c | 33 +++++++++++++++++++++------------
> > >   1 file changed, 21 insertions(+), 12 deletions(-)
> > > 
> > > diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> > > index cfd9fad308a2..ec82441ca3e8 100644
> > > --- a/lib/maple_tree.c
> > > +++ b/lib/maple_tree.c
> > > @@ -4100,23 +4100,32 @@ static inline bool mas_wr_slot_store(struct ma_wr_state *wr_mas)
> > >   {
> > >   	struct ma_state *mas = wr_mas->mas;
> > >   	unsigned char offset = mas->offset;
> > > +	void __rcu **slots = wr_mas->slots;
> > >   	bool gap = false;
> > > -	if (wr_mas->offset_end - offset != 1)
> > > -		return false;
> > > -
> > > -	gap |= !mt_slot_locked(mas->tree, wr_mas->slots, offset);
> > > -	gap |= !mt_slot_locked(mas->tree, wr_mas->slots, offset + 1);
> > > +	gap |= !mt_slot_locked(mas->tree, slots, offset);
> > > +	gap |= !mt_slot_locked(mas->tree, slots, offset + 1);
> > > -	if (mas->index == wr_mas->r_min) {
> > > -		/* Overwriting the range and over a part of the next range. */
> > > -		rcu_assign_pointer(wr_mas->slots[offset], wr_mas->entry);
> > > -		wr_mas->pivots[offset] = mas->last;
> > > -	} else {
> > > -		/* Overwriting a part of the range and over the next range */
> > > -		rcu_assign_pointer(wr_mas->slots[offset + 1], wr_mas->entry);
> > > +	if (wr_mas->offset_end - offset == 1) {
> > > +		if (mas->index == wr_mas->r_min) {
> > > +			/* Overwriting the range and a part of the next one */
> > > +			rcu_assign_pointer(slots[offset], wr_mas->entry);
> > > +			wr_mas->pivots[offset] = mas->last;
> > > +		} else {
> > > +			/* Overwriting a part of the range and the next one */
> > > +			rcu_assign_pointer(slots[offset + 1], wr_mas->entry);
> > > +			wr_mas->pivots[offset] = mas->index - 1;
> > > +			mas->offset++; /* Keep mas accurate. */
> > > +		}
> > > +	} else if (!mt_in_rcu(mas->tree)) {
> > > +		/* Overwriting three ranges, but don't touch the boundaries */
> > 
> > I find this comment misleading.  You actually touch both boundaries for
> > the entry in this case (start and end).  We are just increasing the
> > space in both directions.  You are also not overwriting two of the three
> > entries or ranges, you are expanding one entry in two directions, so
> > both the previous and next ranges will shrink but they will remain. It's
> > more of a "modify three ranges but don't change the outside limits." The
> > similar statement in the commit message should also be changed.
> Yes, your understanding is correct.
> Sorry my comment is not well written, I mean the left boundary of the
> leftmost range and the right boundary of the rightmost range are not
> touched, I will fix it in v2.
> 
> > 
> > Right now, I don't see this code executed by the test program.
> > Inserting a BUG_ON() here and it will not be hit.
> Yes, the current test program does not run to this branch, I will add
> the corresponding test cases in v2.
> 
> > 
> > > +		gap |= !mt_slot_locked(mas->tree, slots, offset + 2);
> > > +		rcu_assign_pointer(slots[offset + 1], wr_mas->entry);
> > >   		wr_mas->pivots[offset] = mas->index - 1;
> > > +		wr_mas->pivots[offset + 1] = mas->last;
> > >   		mas->offset++; /* Keep mas accurate. */
> > > +	} else {
> > 
> > We are hitting this else in check_locky at maple.c:35780 only, I think.
> > You've identified a lack of testing here by the looks of it.
> > 
> > The VMA code does not do this today, and I don't know of any other users
> > which expand/contract entries like this.  Do you think this will be
> > common enough for the optimisation vs a node store?
> I also thought about this problem, but I still regard it as an
> optimization of the slot store. Although it is useless for VMA
> now, I don't know if it will be used in the future. I think that
> if we enter this function, we will most likely enter the first if
> branch now, which will not cause additional overhead and have no
> negative impact, so try to add this case.

Sounds good.  Thanks.

> 
> > 
> > > +		return false;
> > >   	}
> > >   	trace_ma_write(__func__, mas, 0, wr_mas->entry);
> > > -- 
> > > 2.20.1
> > > 
> > >