netdev - Re: [RFC PATCH 10/12] xdp: Add batching support to redirect map

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Message-ID: <20170710195355.40fd9caa@redhat.com>
Date:   Mon, 10 Jul 2017 19:53:55 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, andy@...yhouse.net,
        daniel@...earbox.net, ast@...com, brouer@...hat.com
Subject: Re: [RFC PATCH 10/12] xdp: Add batching support to redirect map

On Fri, 07 Jul 2017 10:37:59 -0700
John Fastabend <john.fastabend@...il.com> wrote:

> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> index 36dc13de..656e334 100644
> --- a/kernel/bpf/devmap.c
> +++ b/kernel/bpf/devmap.c
[...]
>  
> +void __dev_map_insert_ctx(struct bpf_map *map, u32 key)
> +{
> +	struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
> +	unsigned long *bitmap = this_cpu_ptr(dtab->flush_needed);
> +
> +	set_bit(key, bitmap);
> +}

I don't like that this adds an atomic op (set_bit) per packet on a fast-path.
It shows up on a perf top #6 with xdp_redirect_map.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer