netdev - Re: [PATCH v2 bpf-next 4/8] bpf: cpumap: add the possibility to attach an eBPF program to cpumap

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Date:   Tue, 23 Jun 2020 16:56:12 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org, davem@...emloft.net,
        ast@...nel.org, daniel@...earbox.net, toke@...hat.com,
        lorenzo.bianconi@...hat.com, dsahern@...nel.org, brouer@...hat.com
Subject: Re: [PATCH v2 bpf-next 4/8] bpf: cpumap: add the possibility to
 attach an eBPF program to cpumap

On Sat, 20 Jun 2020 00:57:20 +0200
Lorenzo Bianconi <lorenzo@...nel.org> wrote:

> @@ -273,16 +336,20 @@ static int cpu_map_kthread_run(void *data)
>  			prefetchw(page);
>  		}
>  
> -		m = kmem_cache_alloc_bulk(skbuff_head_cache, gfp, n, skbs);
> +		/* Support running another XDP prog on this CPU */
> +		nframes = cpu_map_bpf_prog_run_xdp(rcpu, xdp_frames, n, &stats);
> +

If all frames are dropped my XDP program, then we will call
kmem_cache_alloc_bulk() to allocate zero elements.  I found this during
my testing[1], and I think we should squash my proposed change in[1].

> +		m = kmem_cache_alloc_bulk(skbuff_head_cache, gfp,
> +					  nframes, skbs);
>  		if (unlikely(m == 0)) {
> -			for (i = 0; i < n; i++)
> +			for (i = 0; i < nframes; i++)
>  				skbs[i] = NULL; /* effect: xdp_return_frame */
> -			drops = n;
> +			drops += nframes;
>  		}
>  
>  		local_bh_disable();
> -		for (i = 0; i < n; i++) {
> -			struct xdp_frame *xdpf = frames[i];
> +		for (i = 0; i < nframes; i++) {
> +			struct xdp_frame *xdpf = xdp_frames[i];
>  			struct sk_buff *skb = skbs[i];
>  			int ret;

[1] https://github.com/xdp-project/xdp-project/blob/master/areas/cpumap/cpumap04-map-xdp-prog.org#observations
-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer