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Date: Thu, 27 Jul 2023 09:34:30 -0700
From: Stanislav Fomichev <sdf@...gle.com>
To: Jesper Dangaard Brouer <jbrouer@...hat.com>
Cc: brouer@...hat.com, bpf@...r.kernel.org, ast@...nel.org,
daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev,
song@...nel.org, yhs@...com, john.fastabend@...il.com, kpsingh@...nel.org,
haoluo@...gle.com, jolsa@...nel.org, kuba@...nel.org, toke@...nel.org,
willemb@...gle.com, dsahern@...nel.org, magnus.karlsson@...el.com,
bjorn@...nel.org, maciej.fijalkowski@...el.com, hawk@...nel.org,
netdev@...r.kernel.org, xdp-hints@...-project.net
Subject: Re: [xdp-hints] [RFC net-next v4 2/8] xsk: add TX timestamp and TX
checksum offload support
On 07/27, Jesper Dangaard Brouer wrote:
>
>
> On 26/07/2023 23.25, Stanislav Fomichev wrote:
> > On 07/26, Jesper Dangaard Brouer wrote:
> > >
> > >
> > > On 25/07/2023 01.59, Stanislav Fomichev wrote:
> > > > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > > > index 11652e464f5d..8b40c80557aa 100644
> > > > --- a/include/linux/netdevice.h
> > > > +++ b/include/linux/netdevice.h
> > > > @@ -1660,6 +1660,31 @@ struct xdp_metadata_ops {
> > > > enum xdp_rss_hash_type *rss_type);
> > > > };
> > > > +/*
> > > > + * This structure defines the AF_XDP TX metadata hooks for network devices.
> > > > + * The following hooks can be defined; unless noted otherwise, they are
> > > > + * optional and can be filled with a null pointer.
> > > > + *
> > > > + * int (*tmo_request_timestamp)(void *priv)
> > > > + * This function is called when AF_XDP frame requested egress timestamp.
> > > > + *
> > > > + * int (*tmo_fill_timestamp)(void *priv)
> > > > + * This function is called when AF_XDP frame, that had requested
> > > > + * egress timestamp, received a completion. The hook needs to return
> > > > + * the actual HW timestamp.
> > > > + *
> > > > + * int (*tmo_request_timestamp)(u16 csum_start, u16 csum_offset, void *priv)
> > > > + * This function is called when AF_XDP frame requested HW checksum
> > > > + * offload. csum_start indicates position where checksumming should start.
> > > > + * csum_offset indicates position where checksum should be stored.
> > > > + *
> > > > + */
> > > > +struct xsk_tx_metadata_ops {
> > > > + void (*tmo_request_timestamp)(void *priv);
> > > > + u64 (*tmo_fill_timestamp)(void *priv);
> > > > + void (*tmo_request_checksum)(u16 csum_start, u16 csum_offset, void *priv);
> > > > +};
> > > > +
> > > > /**
> > > > * enum netdev_priv_flags - &struct net_device priv_flags
> > > > *
> > > > @@ -1844,6 +1869,7 @@ enum netdev_ml_priv_type {
> > > > * @netdev_ops: Includes several pointers to callbacks,
> > > > * if one wants to override the ndo_*() functions
> > > > * @xdp_metadata_ops: Includes pointers to XDP metadata callbacks.
> > > > + * @xsk_tx_metadata_ops: Includes pointers to AF_XDP TX metadata callbacks.
> > > > * @ethtool_ops: Management operations
> > > > * @l3mdev_ops: Layer 3 master device operations
> > > > * @ndisc_ops: Includes callbacks for different IPv6 neighbour
> > > > @@ -2100,6 +2126,7 @@ struct net_device {
> > > > unsigned long long priv_flags;
> > > > const struct net_device_ops *netdev_ops;
> > > > const struct xdp_metadata_ops *xdp_metadata_ops;
> > > > + const struct xsk_tx_metadata_ops *xsk_tx_metadata_ops;
> > > > int ifindex;
> > > > unsigned short gflags;
> > > > unsigned short hard_header_len;
> > > > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> > > > index faaba050f843..5febc1a5131e 100644
> > > > --- a/include/linux/skbuff.h
> > > > +++ b/include/linux/skbuff.h
> > > > @@ -581,7 +581,10 @@ struct skb_shared_info {
> > > > /* Warning: this field is not always filled in (UFO)! */
> > > > unsigned short gso_segs;
> > > > struct sk_buff *frag_list;
> > > > - struct skb_shared_hwtstamps hwtstamps;
> > > > + union {
> > > > + struct skb_shared_hwtstamps hwtstamps;
> > > > + struct xsk_tx_metadata *xsk_meta;
> > > > + };
> > > > unsigned int gso_type;
> > > > u32 tskey;
> > > > diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
> > > > index 467b9fb56827..288fa58c4665 100644
> > > > --- a/include/net/xdp_sock.h
> > > > +++ b/include/net/xdp_sock.h
> > > > @@ -90,6 +90,54 @@ int xsk_generic_rcv(struct xdp_sock *xs, struct xdp_buff *xdp);
> > > > int __xsk_map_redirect(struct xdp_sock *xs, struct xdp_buff *xdp);
> > > > void __xsk_map_flush(void);
> > > > +/**
> > > > + * xsk_tx_metadata_request - Evaluate AF_XDP TX metadata at submission
> > > > + * and call appropriate xsk_tx_metadata_ops operation.
> > > > + * @meta: pointer to AF_XDP metadata area
> > > > + * @ops: pointer to struct xsk_tx_metadata_ops
> > > > + * @priv: pointer to driver-private aread
> > > > + *
> > > > + * This function should be called by the networking device when
> > > > + * it prepares AF_XDP egress packet.
> > > > + */
> > > > +static inline void xsk_tx_metadata_request(const struct xsk_tx_metadata *meta,
> > > > + const struct xsk_tx_metadata_ops *ops,
> > > > + void *priv)
> > >
> > > (As you mentioned) this gets inlined in drivers for performance.
> > >
> > > > +{
> > > > + if (!meta)
> > > > + return;
> > > > +
> > > > + if (ops->tmo_request_timestamp)
> > > > + if (meta->flags & XDP_TX_METADATA_TIMESTAMP)
> > > > + ops->tmo_request_timestamp(priv);
> > >
> > > We still have the overhead of function pointer call.
> > > With RETPOLINE this is costly.
> > >
> > > Measured on my testlab CPU E5-1650 v4 @ 3.60GHz
> > > Type:function_call_cost: 3 cycles(tsc) 1.010 ns
> > > Type:func_ptr_call_cost: 30 cycles(tsc) 8.341 ns
> > >
> > > Given this get inlined in drivers, perhaps we can add some
> > > INDIRECT_CALL_1 macros to avoid these indirect calls?
> >
> > I'm assuming that the compiler is smart enough to resolve these const
> > struct ops definitions as long as they are in the same file.
> >
> > At least here is what I see for mlx5e_xmit_xdp_frame_mpwqe: those
> > indirect jumps are resolved and the calls themselves are unrolled.
> > TBF, I don't have retpolines enabled in the config, but I don't think
> > it will bring indirect jumps back? Am I missing anything?
> >
>
> I tried this with CONFIG_RETPOLINE and same results.
> The compiler is smart and inlines the call to mlx5e_xsk_request_checksum().
> This is great, no need for crazy INDIRECT_CALL_1 macros :-)
Perfect, thanks for checking!
> >
> > 0000000000001930 <mlx5e_xmit_xdp_frame_mpwqe>:
> > ; mlx5e_xmit_xdp_frame_mpwqe():
> > ; ./drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c:436
> [...]
> > ; ./drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c:381
> > ; stats->mpwqe++;
> > 1b4a: 49 ff 44 24 08 incq 0x8(%r12)
> > ; ././include/net/xdp_sock.h:107
>
> How do you get objdump to add these file:line annotations?
>
> I use:
> objdump -rS --visualize-jumps=color -Mintel | less -R
I use llvm tools (and complier), so I did:
llvm-objdump -r -S -l --disassemble xxx.o
Most likely it's that -l option? man objdump seems to have it..
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