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

On Sat, Feb 13, 2021 at 6:10 AM Alexander Lobakin [off-list ref] 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@fb.com>