From: Paolo Abeni <pab...@redhat.com> Date: Wed, 4 Apr 2018 14:30:01 +0200
> After commit 694aba690de0 ("ipv4: factorize sk_wmem_alloc updates > done by __ip_append_data()") and commit 1f4c6eb24029 ("ipv6: > factorize sk_wmem_alloc updates done by __ip6_append_data()"), > when transmitting sub MTU datagram, an addtional, unneeded atomic > operation is performed in ip*_append_data() to update wmem_alloc: > in the above condition the delta is 0. > > The above cause small but measurable performance regression in UDP > xmit tput test with packet size below MTU. > > This change avoids such overhead updating wmem_alloc only if > wmem_alloc_delta is non zero. > > The error path is left intentionally unmodified: it's a slow path > and simplicity is preferred to performances. > > Fixes: 694aba690de0 ("ipv4: factorize sk_wmem_alloc updates done by > __ip_append_data()") > Fixes: 1f4c6eb24029 ("ipv6: factorize sk_wmem_alloc updates done by > __ip6_append_data()") > Signed-off-by: Paolo Abeni <pab...@redhat.com> ... > - refcount_add(wmem_alloc_delta, &sk->sk_wmem_alloc); > + if (wmem_alloc_delta) > + refcount_add(wmem_alloc_delta, &sk->sk_wmem_alloc); ... > - refcount_add(wmem_alloc_delta, &sk->sk_wmem_alloc); > + if (wmem_alloc_delta) > + refcount_add(wmem_alloc_delta, &sk->sk_wmem_alloc); This is simple enough, so applied. But I wonder if atomic_{add,sub} and refcount_{add,sub}() should just check for zero inline, just like the {set,clear}_bit() implementations avoid the atomic operation if the bit already has the desired value.