Subject: Re: [xdp-hints] [RFC net-next v4 2/8] xsk: add TX timestamp and TX checksum offload support

From: Stanislav Fomichev <[email-masked]>
Date: Thu, 27 Jul 2023 09:34:30 -0700

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..