netdev - Re: [PATCH net-next v4] net: skb: introduce and use a single page frag cache

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Date:   Wed, 28 Sep 2022 07:24:14 -0700
From:   Alexander H Duyck <alexander.duyck@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexander H Duyck <alexanderduyck@...com>
Subject: Re: [PATCH net-next v4] net: skb: introduce and use a single page
 frag cache

On Wed, 2022-09-28 at 10:43 +0200, Paolo Abeni wrote:
> After commit 3226b158e67c ("net: avoid 32 x truesize under-estimation
> for tiny skbs") we are observing 10-20% regressions in performance
> tests with small packets. The perf trace points to high pressure on
> the slab allocator.
> 
> This change tries to improve the allocation schema for small packets
> using an idea originally suggested by Eric: a new per CPU page frag is
> introduced and used in __napi_alloc_skb to cope with small allocation
> requests.
> 
> To ensure that the above does not lead to excessive truesize
> underestimation, the frag size for small allocation is inflated to 1K
> and all the above is restricted to build with 4K page size.
> 
> Note that we need to update accordingly the run-time check introduced
> with commit fd9ea57f4e95 ("net: add napi_get_frags_check() helper").
> 
> Alex suggested a smart page refcount schema to reduce the number
> of atomic operations and deal properly with pfmemalloc pages.
> 
> Under small packet UDP flood, I measure a 15% peak tput increases.
> 
> Suggested-by: Eric Dumazet <eric.dumazet@...il.com>
> Suggested-by: Alexander H Duyck <alexanderduyck@...com>
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> ---
> v3 -> v4:
>  - NAPI_HAS_SMALL_PAGE <-> 4K page (Eric)
>  - fixed typos in comments (Eric)
> 
> v2 -> v3:
>  - updated Alex email address
>  - fixed build with !NAPI_HAS_SMALL_PAGE_FRAG
> 
> v1 -> v2:
>  - better page_frag_alloc_1k() (Alex & Eric)
>  - avoid duplicate code and gfp_flags misuse in __napi_alloc_skb() (Alex)
> ---


Looks good to me.

Reviewed-by: Alexander Duyck <alexanderduyck@...com>