[xdp-hints] Re: [PATCH bpf-next v8 4/5] igc: Refactor empty packet insertion into a reusable function

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Wed, Feb 05, 2025 at 03:43:19PM +0100, Song, Yoong Siang wrote:
> On Wednesday, February 5, 2025 8:31 PM, Fijalkowski, Maciej <maciej.fijalkowski@intel.com> wrote:
> >On Wed, Feb 05, 2025 at 10:41:15AM +0800, Song Yoong Siang wrote:
> >> Refactor the code for inserting an empty packet into a new function
> >> igc_insert_empty_packet(). This change extracts the logic for inserting
> >> an empty packet from igc_xmit_frame_ring() into a separate function,
> >> allowing it to be reused in future implementations, such as the XDP
> >> zero copy transmit function.
> >>
> >> This patch introduces no functional changes.
> >>
> >> Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
> >
> >Your SoB should be last in the set of tags.
> >
> 
> Noted. Thanks for the tips.
> 
> >> Reviewed-by: Faizal Rahim <faizal.abdul.rahim@linux.intel.com>
> >> ---
> >>  drivers/net/ethernet/intel/igc/igc_main.c | 42 ++++++++++++-----------
> >>  1 file changed, 22 insertions(+), 20 deletions(-)
> >>
> >> diff --git a/drivers/net/ethernet/intel/igc/igc_main.c
> >b/drivers/net/ethernet/intel/igc/igc_main.c
> >> index 21f318f12a8d..553d6d82af0d 100644
> >> --- a/drivers/net/ethernet/intel/igc/igc_main.c
> >> +++ b/drivers/net/ethernet/intel/igc/igc_main.c
> >> @@ -1566,6 +1566,26 @@ static bool igc_request_tx_tstamp(struct igc_adapter
> >*adapter, struct sk_buff *s
> >>  	return false;
> >>  }
> >>
> >> +static void igc_insert_empty_packet(struct igc_ring *tx_ring)
> >> +{
> >> +	struct igc_tx_buffer *empty_info;
> >> +	struct sk_buff *empty;
> >> +	void *data;
> >> +
> >> +	empty_info = &tx_ring->tx_buffer_info[tx_ring->next_to_use];
> >> +	empty = alloc_skb(IGC_EMPTY_FRAME_SIZE, GFP_ATOMIC);
> >> +	if (!empty)
> >> +		return;
> >> +
> >> +	data = skb_put(empty, IGC_EMPTY_FRAME_SIZE);
> >> +	memset(data, 0, IGC_EMPTY_FRAME_SIZE);
> >> +
> >> +	igc_tx_ctxtdesc(tx_ring, 0, false, 0, 0, 0);
> >> +
> >> +	if (igc_init_tx_empty_descriptor(tx_ring, empty, empty_info) < 0)
> >> +		dev_kfree_skb_any(empty);
> >
> >I still don't like the fact igc_insert_empty_packet() doesn't communicate
> >to caller whether it successfully produced descriptors or not.
> >
> >Look at this from igc_xmit_frame_ring() POV:
> >- at the beginning you peek at Tx ring whether there is required amount of
> >  descriptors free to be used
> >- but then here's your additional routine which might consume two more
> >  descs and you are not aware of the status
> >- then you continue to further produce descriptors assuming there is
> >  enough space in Tx ring
> >
> >Right now igc_init_tx_empty_descriptor() returns -EBUSY when ring is full.
> >How can that happen in the first place + what if it would *really* happen
> >though? You just continue with your Tx flow.
> >
> >What I'm trying to say here is, at least from correctness POV, you should
> >take into the account two potential descriptors for launchtime feature
> >when calling igc_maybe_stop_tx(). And igc_init_tx_empty_descriptor()
> >should not really care about space in ring, it should be a caller's job to
> >call it only when it will be sure it's safe to do so.
> >
> 
> Agree with you.
> 
> In db0b124f02ba ("igc: Enhance Qbv scheduling by using first flag bit"),
> the 2 descriptors needed by empty packet is already taken into
> consideration by changing igc_maybe_stop_tx(tx_ring, count + 3) to
> igc_maybe_stop_tx(tx_ring, count + 5), so not enough ring space issue will not
> happened. However, the comment session is not updated, maybe i can update
> it in next version of this patch as below:
> 
> @@ -1586,6 +1608,7 @@ static netdev_tx_t igc_xmit_frame_ring(struct sk_buff *skb,
>          *      + 1 desc for skb_headlen/IGC_MAX_DATA_PER_TXD,
>          *      + 2 desc gap to keep tail from touching head,
>          *      + 1 desc for context descriptor,
> +        *      + 2 desc for inserting an empty packet for launch time,
>          * otherwise try next time
>          */
>         for (f = 0; f < skb_shinfo(skb)->nr_frags; f++)

Ahh good then, I didn't pay enough attention to comment. So it meant that
first two entries from comment were covered by @count and 2 desc gap + 1
ctxt desc were behind '3', previously.

> 
> Since number of descriptor needed is guaranteed. I will take out the
> igc_desc_unused() checking in igc_init_tx_empty_descriptor().

Yes

> However, empty packet insertion might fail due to skb allocation failure
> and DMA mapping error. How about i make sure skb allocation and DMA
> mapping working, before proceed to fill in igc_tx_buffer, context desc,
> and data desc?
> IMHO, because these two errors are unlikely to happen, print a kernel
> warning msg should be enough.
> 
> @@ -1108,20 +1108,12 @@ static int igc_init_empty_frame(struct igc_ring *ring,
>         return 0;
>  }
> 
> -static int igc_init_tx_empty_descriptor(struct igc_ring *ring,
> +static void igc_init_tx_empty_descriptor(struct igc_ring *ring,
>                                         struct sk_buff *skb,
>                                         struct igc_tx_buffer *first)
>  {
>         union igc_adv_tx_desc *desc;
>         u32 cmd_type, olinfo_status;
> -       int err;
> -
> -       if (!igc_desc_unused(ring))
> -               return -EBUSY;
> -
> -       err = igc_init_empty_frame(ring, first, skb);
> -       if (err)
> -               return err;
> 
>         cmd_type = IGC_ADVTXD_DTYP_DATA | IGC_ADVTXD_DCMD_DEXT |
>                    IGC_ADVTXD_DCMD_IFCS | IGC_TXD_DCMD |
> @@ -1140,8 +1132,6 @@ static int igc_init_tx_empty_descriptor(struct igc_ring *ring,
>         ring->next_to_use++;
>         if (ring->next_to_use == ring->count)
>                 ring->next_to_use = 0;
> -
> -       return 0;
>  }
> 
>  #define IGC_EMPTY_FRAME_SIZE 60
> @@ -1567,6 +1557,38 @@ static bool igc_request_tx_tstamp(struct igc_adapter *adapter, struct sk_buff *s
>         return false;
>  }
> 
> +static void igc_insert_empty_packet(struct igc_ring *tx_ring)
> +{
> +       struct igc_tx_buffer *empty_info;
> +       struct sk_buff *empty;
> +       void *data;
> +
> +       empty_info = &tx_ring->tx_buffer_info[tx_ring->next_to_use];
> +       empty = alloc_skb(IGC_EMPTY_FRAME_SIZE, GFP_ATOMIC);
> +       if (unlikely(!empty)) {
> +               netdev_warn(tx_ring->netdev,
> +                           "Fail to alloc skb for empty packet\n");

That should be at least ratelimited I think, but what really is the
problem to make these routines return status and check them on caller
side?

> +               return;
> +       }
> +
> +       data = skb_put(empty, IGC_EMPTY_FRAME_SIZE);
> +       memset(data, 0, IGC_EMPTY_FRAME_SIZE);
> +
> +       /* Prepare DMA mapping and Tx buffer information */
> +       if (unlikely(igc_init_empty_frame(tx_ring, empty_info, empty))) {
> +               dev_kfree_skb_any(empty);
> +               netdev_warn(tx_ring->netdev,
> +                           "Fail to map DMA for empty packet\n");
> +               return;
> +       }
> +
> +       /* Prepare context descriptor for empty packet */
> +       igc_tx_ctxtdesc(tx_ring, 0, false, 0, 0, 0);
> +
> +       /* Prepare data descriptor for empty packet */
> +       igc_init_tx_empty_descriptor(tx_ring, empty, empty_info);
> +}
> +
>  static netdev_tx_t igc_xmit_frame_ring(struct sk_buff *skb,
>                                        struct igc_ring *tx_ring)
>  {
> 
> Are above codes resolve your concern? If yes, i can test and add
> them into this patch in next version submission.
> 
> Thanks & Regards
> Siang