Re: [PATCH v6 net-next 00/11] skbuff: introduce skbuff_heads bulking and reusing

[Alexander Duyck]

On Sat, Feb 13, 2021 at 6:10 AM Alexander Lobakin <alobakin@xxxxx> wrote:
>
> Currently, all sorts of skb allocation always do allocate
> skbuff_heads one by one via kmem_cache_alloc().
> On the other hand, we have percpu napi_alloc_cache to store
> skbuff_heads queued up for freeing and flush them by bulks.
>
> We can use this cache not only for bulk-wiping, but also to obtain
> heads for new skbs and avoid unconditional allocations, as well as
> for bulk-allocating (like XDP's cpumap code and veth driver already
> do).
>
> As this might affect latencies, cache pressure and lots of hardware
> and driver-dependent stuff, this new feature is mostly optional and
> can be issued via:
>  - a new napi_build_skb() function (as a replacement for build_skb());
>  - existing {,__}napi_alloc_skb() and napi_get_frags() functions;
>  - __alloc_skb() with passing SKB_ALLOC_NAPI in flags.
>
> iperf3 showed 35-70 Mbps bumps for both TCP and UDP while performing
> VLAN NAT on 1.2 GHz MIPS board. The boost is likely to be bigger
> on more powerful hosts and NICs with tens of Mpps.
>
> Note on skbuff_heads from distant slabs or pfmemalloc'ed slabs:
>  - kmalloc()/kmem_cache_alloc() itself allows by default allocating
>    memory from the remote nodes to defragment their slabs. This is
>    controlled by sysctl, but according to this, skbuff_head from a
>    remote node is an OK case;
>  - The easiest way to check if the slab of skbuff_head is remote or
>    pfmemalloc'ed is:
>
>         if (!dev_page_is_reusable(virt_to_head_page(skb)))
>                 /* drop it */;
>
>    ...*but*, regarding that most slabs are built of compound pages,
>    virt_to_head_page() will hit unlikely-branch every single call.
>    This check costed at least 20 Mbps in test scenarios and seems
>    like it'd be better to _not_ do this.

<snip>

> Alexander Lobakin (11):
>   skbuff: move __alloc_skb() next to the other skb allocation functions
>   skbuff: simplify kmalloc_reserve()
>   skbuff: make __build_skb_around() return void
>   skbuff: simplify __alloc_skb() a bit
>   skbuff: use __build_skb_around() in __alloc_skb()
>   skbuff: remove __kfree_skb_flush()
>   skbuff: move NAPI cache declarations upper in the file
>   skbuff: introduce {,__}napi_build_skb() which reuses NAPI cache heads
>   skbuff: allow to optionally use NAPI cache from __alloc_skb()
>   skbuff: allow to use NAPI cache from __napi_alloc_skb()
>   skbuff: queue NAPI_MERGED_FREE skbs into NAPI cache instead of freeing
>
>  include/linux/skbuff.h |   4 +-
>  net/core/dev.c         |  16 +-
>  net/core/skbuff.c      | 428 +++++++++++++++++++++++------------------
>  3 files changed, 242 insertions(+), 206 deletions(-)
>

With the last few changes and testing to verify the need to drop the
cache clearing this patch set looks good to me.

Reviewed-by: Alexander Duyck <alexanderduyck@xxxxxx>