Re: [Intel-wired-lan] [PATCH RFC net-next v4 6/9] iavf: switch to Page Pool

[Alexander Lobakin]

From: Alexander Duyck <alexander.duyck@xxxxxxxxx>
Date: Thu, 6 Jul 2023 10:28:06 -0700

> On Thu, Jul 6, 2023 at 9:57 AM Alexander Lobakin
> <aleksander.lobakin@xxxxxxxxx> wrote:
>>
>> From: Alexander Duyck <alexander.duyck@xxxxxxxxx>
>> Date: Thu, 6 Jul 2023 08:26:00 -0700
>>
>>> On Wed, Jul 5, 2023 at 8:58 AM Alexander Lobakin
>>> <aleksander.lobakin@xxxxxxxxx> wrote:
>>>>
>>>> Now that the IAVF driver simply uses dev_alloc_page() + free_page() with
>>>> no custom recycling logics, it can easily be switched to using Page
>>>> Pool / libie API instead.
>>>> This allows to removing the whole dancing around headroom, HW buffer
>>>> size, and page order. All DMA-for-device is now done in the PP core,
>>>> for-CPU -- in the libie helper.
>>>> Use skb_mark_for_recycle() to bring back the recycling and restore the
>>>> performance. Speaking of performance: on par with the baseline and
>>>> faster with the PP optimization series applied. But the memory usage for
>>>> 1500b MTU is now almost 2x lower (x86_64) thanks to allocating a page
>>>> every second descriptor.
>>>>
>>>> Signed-off-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
>>>
>>> One thing I am noticing is that there seems to be a bunch of cleanup
>>> changes in here as well. Things like moving around values within
>>> structures which I am assuming are to fill holes. You may want to look
>>> at breaking some of those out as it makes it a bit harder to review
>>> this since they seem like unrelated changes.
>>
>> min_mtu and watchdog are unrelated, I'll drop those.
>> Moving tail pointer around was supposed to land in a different commit,
>> not this one, as I wrote 10 minutes ago already :s
>>
>> [...]
>>
>>>> -       bi_size = sizeof(struct iavf_rx_buffer) * rx_ring->count;
>>>> -       memset(rx_ring->rx_bi, 0, bi_size);
>>>> -
>>>> -       /* Zero out the descriptor ring */
>>>> -       memset(rx_ring->desc, 0, rx_ring->size);
>>>> -
>>>
>>> I have some misgivings about not clearing these. We may want to double
>>> check to verify the code paths are resilient enough that it won't
>>> cause any issues w/ repeated up/down testing on the interface. The
>>> general idea is to keep things consistent w/ the state after
>>> setup_rx_descriptors. If we don't need this when we don't need to be
>>> calling the zalloc or calloc version of things in
>>> setup_rx_descriptors.
>>
>> Both arrays will be freed couple instructions below, why zero them?
> 
> Ugh. You are right, but not for a good reason. So the other Intel
> drivers in the past would be doing the clean_rx_ring calls on the
> _down() with the freeing of resources on _close(). Specifically it
> allowed reducing the overhead for things like resets or setting
> changes since it didn't require reallocating the descriptor rings and
> buffer info structures.
> 
> I guess you are good to remove these since this code doesn't do that.

We might go back to this to not always do a full circle when not needed,
but currently this is redundant.

> 
>>>
>>>
>>>>         rx_ring->next_to_clean = 0;
>>>>         rx_ring->next_to_use = 0;
>>>>  }
>>
>> [...]
>>
>>>>         struct net_device *netdev;      /* netdev ring maps to */
>>>>         union {
>>>> +               struct libie_rx_buffer *rx_bi;
>>>>                 struct iavf_tx_buffer *tx_bi;
>>>> -               struct iavf_rx_buffer *rx_bi;
>>>>         };
>>>>         DECLARE_BITMAP(state, __IAVF_RING_STATE_NBITS);
>>>> +       u8 __iomem *tail;
>>>>         u16 queue_index;                /* Queue number of ring */
>>>>         u8 dcb_tc;                      /* Traffic class of ring */
>>>> -       u8 __iomem *tail;
>>>>
>>>>         /* high bit set means dynamic, use accessors routines to read/write.
>>>>          * hardware only supports 2us resolution for the ITR registers.
>>>
>>> I'm assuming "tail" was moved here since it is a pointer and fills a hole?
>>
>> (see above)
>>
>>>
>>>> @@ -329,9 +264,8 @@ struct iavf_ring {
>>>>          */
>>>>         u16 itr_setting;
>>>>
>>>> -       u16 count;                      /* Number of descriptors */
>>>>         u16 reg_idx;                    /* HW register index of the ring */
>>>> -       u16 rx_buf_len;
>>>> +       u16 count;                      /* Number of descriptors */
>>>
>>> Why move count down here? It is moving the constant value that is
>>> read-mostly into an area that will be updated more often.
>>
>> With the ::tail put in a different slot, ::count was landing in a
>> different cacheline. I wanted to avoid this. But now I feel like I was
>> just lazy and must've tested both variants to see if this move affects
>> performance. I'll play with this one in the next rev.
> 
> The performance impact should be minimal. Odds are the placement was
> the way it was since it was probably just copying the original code
> that has been there since igb/ixgbe. The general idea is just keep the
> read-mostly items grouped at the top and try to order them somewhat by
> frequency of being read so that wherever the cache line ends up you
> won't take much of a penalty as hopefully you will just have the
> infrequently read items end up getting pulled into the active cache
> line.

+

> 
>>>
>>>>         /* used in interrupt processing */
>>>>         u16 next_to_use;
>>>> @@ -398,17 +332,6 @@ struct iavf_ring_container {
>>>>  #define iavf_for_each_ring(pos, head) \
>>>>         for (pos = (head).ring; pos != NULL; pos = pos->next)
>>>>
>>>> -static inline unsigned int iavf_rx_pg_order(struct iavf_ring *ring)
>>>> -{
>>>> -#if (PAGE_SIZE < 8192)
>>>> -       if (ring->rx_buf_len > (PAGE_SIZE / 2))
>>>> -               return 1;
>>>> -#endif
>>>> -       return 0;
>>>> -}
>>>> -
>>>> -#define iavf_rx_pg_size(_ring) (PAGE_SIZE << iavf_rx_pg_order(_ring))
>>>> -
>>>
>>> All this code probably could have been removed in an earlier patch
>>> since I don't think we need the higher order pages once we did away
>>> with the recycling. Odds are we can probably move this into the
>>> recycling code removal.
>>
>> This went here as I merged "always use order 0" commit with "switch to
>> Page Pool". In general, IIRC having removals of all the stuff at once in
>> one commit (#2) was less readable than the current version, but I'll
>> double-check.
> 
> It all depends on how much is having to be added to accommodate this.
> In my mind when we did away with the page splitting/recycling we also
> did away with the need for the higher order pages. That is why I was
> thinking it might make more sense there as it would just be more
> removals with very few if any additions needed to support it.
Yeah, I'll try and see whether any pieces can be grouped differently for
better reading/logics.

[...]

Thanks!
Olek