On Tue, Mar 23, 2021 at 04:55:31PM +0000, Alexander Lobakin wrote:

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Thanks for the testing!
Any chance you can get a perf measurement on this?

I guess you mean perf-report (--stdio) output, right?

Yea,
As hinted below, I am just trying to figure out if on Alexander's platform the
cost of syncing, is bigger that free-allocate. I remember one armv7 were that
was the case.

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Is DMA syncing taking a substantial amount of your cpu usage?

(+1 this is an important question)

Sure, I'll drop perf tools to my test env and share the results,
maybe tomorrow or in a few days.
From what I know for sure about MIPS and my platform,
post-Rx synching (dma_sync_single_for_cpu()) is a no-op, and
pre-Rx (dma_sync_single_for_device() etc.) is a bit expensive.
I always have sane page_pool->pp.max_len value (smth about 1668
for MTU of 1500) to minimize the overhead.

By the word, IIRC, all machines shipped with mvpp2 have hardware
cache coherency units and don't suffer from sync routines at all.
That may be the reason why mvpp2 wins the most from this series.

Yep exactly. It's also the reason why you explicitly have to opt-in using the
recycling (by marking the skb for it), instead of hiding the feature in the
page pool internals 

Cheers
/Ilias

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[0] https://lore.kernel.org/netdev/20210323153550.130385-1-alobakin@pm.me (local)

That would be the same as for mvneta:

Overhead  Shared Object     Symbol
  24.10%  [kernel]          [k] __pi___inval_dcache_area
  23.02%  [mvneta]          [k] mvneta_rx_swbm
   7.19%  [kernel]          [k] kmem_cache_alloc

Anyway, I tried to use the recycling *and* napi_build_skb on mvpp2,
and I get lower packet rate than recycling alone.
I don't know why, we should investigate it.

mvpp2 driver doesn't use napi_consume_skb() on its Tx completion path.
As a result, NAPI percpu caches get refilled only through
kmem_cache_alloc_bulk(), and most of skbuff_head recycling
doesn't work.

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Regards,
--
per aspera ad upstream

Oh, I love that one!

Al