On Wed, 2017-05-31 at 10:04 -0700, Eric Dumazet wrote: > On Mon, 2017-05-29 at 17:27 +0200, Paolo Abeni wrote: > > when udp_recvmsg() is executed, on x86_64 and other archs, most skb > > fields are on cold cachelines. > > If the skb are linear and the kernel don't need to compute the udp > > csum, only a handful of skb fields are required by udp_recvmsg(). > > Since we already use skb->dev_scratch to cache hot data, and > > there are 32 bits unused on 64 bit archs, use such field to cache > > as much data as we can, and try to prefetch on dequeue the relevant > > fields that are left out. > > > > This can save up to 2 cache miss per packet. > > okay ;) > > > > > Signed-off-by: Paolo Abeni <pab...@redhat.com> > > --- > > net/ipv4/udp.c | 114 > > +++++++++++++++++++++++++++++++++++++++++++++++++++------ > > 1 file changed, 103 insertions(+), 11 deletions(-) > > > > diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c > > index 53fa48d..616132e 100644 > > --- a/net/ipv4/udp.c > > +++ b/net/ipv4/udp.c > > @@ -1163,6 +1163,83 @@ int udp_sendpage(struct sock *sk, struct page *page, > > int offset, > > return ret; > > } > > > > +/* Copy as much information as possible into skb->dev_scratch to avoid > > + * possibly multiple cache miss on dequeue(); > > + */ > > +#if BITS_PER_LONG == 64 > > + > > +/* we can store multiple info here: truesize, len and the bit needed to > > + * compute skb_csum_unnecessary will be on cold cache lines at recvmsg > > + * time. > > + * skb->len can be stored on 16 bits since the udp header has been already > > + * validated and pulled. > > + */ > > +struct udp_dev_scratch { > > + __u32 truesize; > > + __u16 len; > > + __u16 is_linear:1; > > + __u16 csum_unnecessary:1; > > What about > u32 truesize; > u16 len; > bool is_linear; > bool csum_unnecessary; > > I do not believe the __ prefix is necessary for a local structure (not > uapi) > > Also a plain bool or u8 is faster than a bit field (shorter > instructions)
Thank you! I like the above! I'll go for 'bool' usage in v2, Paolo p.s. I used the bitfield because I initially had an additional, very ugly, patch saving another cache miss and requiring one more bit there, but said patch hurted so much the sight that I had to drop it.