netdev - Re: [PATCH v8 bpf-next 01/14] xdp: introduce mb in xdp_buff/xdp

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Message-ID: <YIq6eoS+5UZRTi/5@lore-desk>
Date:   Thu, 29 Apr 2021 15:54:02 +0200
From:   Lorenzo Bianconi <lorenzo@...nel.org>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        lorenzo.bianconi@...hat.com, davem@...emloft.net, kuba@...nel.org,
        ast@...nel.org, daniel@...earbox.net, shayagr@...zon.com,
        sameehj@...zon.com, john.fastabend@...il.com, dsahern@...nel.org,
        echaudro@...hat.com, jasowang@...hat.com,
        alexander.duyck@...il.com, saeed@...nel.org,
        maciej.fijalkowski@...el.com
Subject: Re: [PATCH v8 bpf-next 01/14] xdp: introduce mb in xdp_buff/xdp_frame

> >  static __always_inline void
> > @@ -116,7 +120,8 @@ struct xdp_frame {
> >  	u16 len;
> >  	u16 headroom;
> >  	u32 metasize:8;
> > -	u32 frame_sz:24;
> > +	u32 frame_sz:23;
> > +	u32 mb:1; /* xdp non-linear frame */
> >  	/* Lifetime of xdp_rxq_info is limited to NAPI/enqueue time,
> >  	 * while mem info is valid on remote CPU.
> >  	 */
> 
> So, it seems that these bitfield's are the root-cause of the
> performance regression.  Credit to Alexei whom wisely already point
> this out[1] in V2 ;-)
> 
> [1] https://lore.kernel.org/netdev/20200904010705.jm6dnuyj3oq4cpjd@ast-mbp.dhcp.thefacebook.com/

yes, shame on me..yesterday I recalled email from Alexei debugging the issue
reported by Magnus.
In the current approach I am testing (not posted upstream yet) I reduced the
size of xdp_mem_info as proposed by Jesper in [0] and I added a flags field
in xdp_frame/xdp_buff we can use for multiple features (e.g. multi-buff or hw csum
hints). Doing so, running xdp_rxq_info sample on ixgbe 10Gbps NIC I do not have any
performance regressions for xdp_tx or xdp_drop. Same results have been reported by
Magnus off-list on i40e (we have a 1% regression on xdp_sock tests iiuc).
I will continue working on this.

Regards,
Lorenzo

[0] https://patchwork.kernel.org/project/netdevbpf/patch/20210409223801.104657-2-mcroce@linux.microsoft.com/

> 
> 
> > @@ -179,6 +184,7 @@ void xdp_convert_frame_to_buff(struct xdp_frame *frame, struct xdp_buff *xdp)
> >  	xdp->data_end = frame->data + frame->len;
> >  	xdp->data_meta = frame->data - frame->metasize;
> >  	xdp->frame_sz = frame->frame_sz;
> > +	xdp->mb = frame->mb;
> >  }
> >  
> >  static inline
> > @@ -205,6 +211,7 @@ int xdp_update_frame_from_buff(struct xdp_buff *xdp,
> >  	xdp_frame->headroom = headroom - sizeof(*xdp_frame);
> >  	xdp_frame->metasize = metasize;
> >  	xdp_frame->frame_sz = xdp->frame_sz;
> > +	xdp_frame->mb = xdp->mb;
> >  
> >  	return 0;
> >  }
> 
> -- 
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer
> 

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