From: Alexander H Duyck <redacted>
Date: Tue, 30 May 2023 09:18:40 -0700

On Thu, 2023-05-25 at 14:57 +0200, Alexander Lobakin wrote:

quoted

The Rx hotpath code of IAVF is not well-optimized TBH. Before doing any
further buffer model changes, shake it up a bit. Notably:

1. Cache more variables on the stack.
   DMA device, Rx page size, NTC -- these are the most common things
   used all throughout the hotpath, often in loops on each iteration.
   Instead of fetching (or even calculating, as with the page size) them
   from the ring all the time, cache them on the stack at the beginning
   of the NAPI polling callback. NTC will be written back at the end,
   the rest are used read-only, so no sync needed.

The advantage of this is going to vary based on the attribute. One of
the reasons why I left most of this on the ring is because the section
of the ring most of these variables were meant to be read-mostly and
shouldn't have resulted in any additional overhead versus accessing
them from the stack.

But not all of these variables are read-only. E.g. NTC is often
modified. Page size was calculated per descriptor, but could be once a
poll cycle starts, and so on.

quoted

2. Don't move the recycled buffers around the ring.
   The idea of passing the page of the right-now-recycled-buffer to a
   different buffer, in this case, the first one that needs to be
   allocated, moreover, on each new frame, is fundamentally wrong. It
   involves a few o' fetches, branches and then writes (and one Rx
   buffer struct is at least 32 bytes) where they're completely unneeded,
   but gives no good -- the result is the same as if we'd recycle it
   inplace, at the same position where it was used. So drop this and let
   the main refilling function take care of all the buffers, which were
   processed and now need to be recycled/refilled.

The next_to_alloc logic was put in place to deal with systems that are
experiencing memory issues. Specifically what can end up happening is
that the ring can stall due to failing memory allocations and the
memory can get stuck on the ring. For that reason we were essentially
defragmenting the buffers when we started suffering memory pressure so
that they could be reusued and/or freed following immediate use.

Basically what you are trading off is some exception handling for
performance by removing it.

I'm not sure this helps a lot, but OTOH this really slows down things,
esp. given that this code is run all the time, not only when a memory
allocation fail happens.

quoted

3. Don't allocate with %GPF_ATOMIC on ifup.
   This involved introducing the @gfp parameter to a couple functions.
   Doesn't change anything for Rx -> softirq.

Any specific reason for this? Just wondering if this is meant to
address some sort of memory pressure issue since it basically just
means the allocation can go out and try to free other memory.

Yes, I'm no MM expert, but I've seen plenty of times messages from the
MM folks that ATOMIC shouldn't be used in non-atomic contexts. Atomic
allocation is able to grab memory from some sort of critical reservs and
all that, and the less we touch them, the better. Outside of atomic
contexts they should not be touched.

quoted

4. 1 budget unit == 1 descriptor, not skb.
   There could be underflow when receiving a lot of fragmented frames.
   If each of them would consist of 2 frags, it means that we'd process
   64 descriptors at the point where we pass the 32th skb to the stack.
   But the driver would count that only as a half, which could make NAPI
   re-enable interrupts prematurely and create unnecessary CPU load.

Not sure I agree with this. The problem is the overhead for an skb
going up the stack versus a fragment are pretty signficant. Keep in
mind that most of the overhead for a single buffer occurs w/
napi_gro_receive and is not actually at the driver itself. The whole
point of the budget is to meter out units of work, not to keep you in
the busy loop. This starts looking like the old code where the Intel
drivers were returning either budget or 0 instead of supporting the
middle ground.

No, certainly not this :D

The point of budget is to limit the amount of time drivers can spend on
cleaning their rings. Making skb the unit makes the unit very logical
and flexible, but I'd say it should always be solid. Imagine you get a
frame which got spanned across 5 buffers. You spend x5 time (roughly) to
build an skb and pass it up the stack vs when you get a linear frame in
one buffer, but according to your logics both of these cases count as 1
unit, while the amount of time spent differs significantly. I can't say
that's fair enough.

quoted

5. Shortcut !size case.
   It's super rare, but possible -- for example, if the last buffer of
   the fragmented frame contained only FCS, which was then stripped by
   the HW. Instead of checking for size several times when processing,
   quickly reuse the buffer and jump to the skb fields part.
6. Refill the ring after finishing the polling loop.
   Previously, the loop wasn't starting a new iteration after the 64th
   desc, meaning that we were always leaving 16 buffers non-refilled
   until the next NAPI poll. It's better to refill them while they're
   still hot, so do that right after exiting the loop as well.
   For a full cycle of 64 descs, there will be 4 refills of 16 descs
   from now on.

Function: add/remove: 4/2 grow/shrink: 0/5 up/down: 473/-647 (-174)

+ up to 2% performance.

What is the test you saw the 2% performance improvement in? Is it
something XDP related or a full stack test?

Not XDP, it's not present in this driver at this point :D
Stack test, but without usercopy overhead. Trafgen bombs the NIC, the
driver builds skbs and passes it up the stack, the stack does GRO etc,
and then the frames get dropped on IP input because there's no socket.

quoted

Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>

Also one thing I am not a huge fan of is a patch that is really a
patchset onto itself. With all 6 items called out here I would have
preferred to see this as 6 patches as it would have been easier to
review.

Agree BTW, I'm not a fan of this patch either. I wasn't sure what to do
with it, as splitting it into 6 explodes the series into a monster, but
proceeding without it increases diffstat and complicates things later
on. I'll try the latter, but will see. 17 patches is not the End of Days
after all.

quoted

---
 drivers/net/ethernet/intel/iavf/iavf_main.c |   2 +-
 drivers/net/ethernet/intel/iavf/iavf_txrx.c | 259 +++++++++-----------
 drivers/net/ethernet/intel/iavf/iavf_txrx.h |   3 +-
 3 files changed, 114 insertions(+), 150 deletions(-)

[...]

quoted

@@ -943,23 +945,17 @@ bool iavf_alloc_rx_buffers(struct iavf_ring *rx_ring, u16 cleaned_count)
 
 		/* clear the status bits for the next_to_use descriptor */
 		rx_desc->wb.qword1.status_error_len = 0;
-
-		cleaned_count--;
-	} while (cleaned_count);
+	} while (--to_refill);

Just a nit. You might want to break this up into two statements like I
had before. I know some people within Intel weren't a huge fan of when
I used to do that kind of thing all the time in loops where I would do
the decrement and test in one line.. :)

Should I please them or do it as I want to? :D I realize from the
compiler's PoV it's most likely the same, but dunno, why not.

quoted

 
 	if (rx_ring->next_to_use != ntu)
 		iavf_release_rx_desc(rx_ring, ntu);
 
-	return false;

[...]

quoted

 	/* if we are the last buffer then there is nothing else to do */
 #define IAVF_RXD_EOF BIT(IAVF_RX_DESC_STATUS_EOF_SHIFT)
 	if (likely(iavf_test_staterr(rx_desc, IAVF_RXD_EOF)))

You may want to see if you can get rid of this function entirely,
perhaps you do in a later patch. This function was added for ixgbe back
in the day to allow us to place the skb back in the ring for the RSC
based workloads where we had to deal with interleaved frames in the Rx
path.

For example, one question here would be why are we passing skb? It
isn't used as far as I can tell.

Yes, I'm optimizing all this out later in the series. I was surprised
just as much as you when I saw skb getting passed to do nothing ._.

[...]

Thanks for the detailed reviews, stuff that Intel often lacks :s :D

Olek