Subject: Re: [net-next V2 PATCH 1/5] bpf: introduce new bpf cpu map type BPF_MAP_TYPE_CPUMAP

From: Jakub Kicinski <[email-masked]>
Date: Fri, 29 Sep 2017 11:41:54 -0700

On Fri, 29 Sep 2017 18:34:12 +0200, Jesper Dangaard Brouer wrote:
> The 'cpumap' is primary used as a backend map for XDP BPF helper
> call bpf_redirect_map() and XDP_REDIRECT action, like 'devmap'.
> 
> This patch implement the main part of the map.  It is not connected to
> the XDP redirect system yet, and no SKB allocation are done yet.
> 
> The main concern in this patch is to ensure the datapath can run
> without any locking.  This adds complexity to the setup and tear-down
> procedure, which assumptions are extra carefully documented in the
> code comments.
> 
> V2: make sure array isn't larger than num possible CPUs
> 
> Signed-off-by: Jesper Dangaard Brouer <[email-masked]>

Few trivial nitpicks, hope you don't mind :)

> @@ -0,0 +1,555 @@
> +/* bpf/cpumap.c
> + *
> + * Copyright (c) 2017 Jesper Dangaard Brouer, Red Hat Inc.
> + * Released under terms in GPL version 2.  See COPYING.
> + */
> +
> +/* The 'cpumap' is primary used as a backend map for XDP BPF helper
> + * call bpf_redirect_map() and XDP_REDIRECT action, like 'devmap'.
> + *
> + * Unlike devmap which redirect XDP frames out another NIC device,
> + * this map type redirect raw XDP frames to another CPU.  The remote
> + * CPU will do SKB-allocation and call the normal network stack.
> + *
> + * This is a scalability and isolation mechanism, that allow
> + * separating the early driver network XDP layer, from the rest of the
> + * netstack, and assigning dedicated CPUs for this stage.  This
> + * basically allows for 10G wirespeed pre-filtering via bpf.
> + */
> +#include <linux/bpf.h>
> +#include <linux/filter.h>
> +#include <linux/ptr_ring.h>
> +
> +#include <linux/sched.h>
> +#include <linux/workqueue.h>
> +#include <linux/kthread.h>
> +
> +/*
> + * General idea: XDP packets getting XDP redirected to another CPU,
> + * will maximum be stored/queued for one driver ->poll() call.  It is
> + * guaranteed that setting flush bit and flush operation happen on
> + * same CPU.  Thus, cpu_map_flush operation can deduct via this_cpu_ptr()
> + * which queue in bpf_cpu_map_entry contains packets.
> + */
> +
> +#define CPU_MAP_BULK_SIZE 8  /* 8 == one cacheline on 64-bit archs */
> +struct xdp_bulk_queue {
> +	void *q[CPU_MAP_BULK_SIZE];
> +	unsigned int count;
> +};

Out of curiosity - would it make sense to make sure the entire struct
fits into a cache line?  The comment seems to indicate that the array is
sized to fit a cache line, but then there is also the count member...

> +/*
> + * After xchg pointer to bpf_cpu_map_entry, use the call_rcu() to
...

There is a mix for networking and non-networking style comments in this
file, is this intentional?

> +const struct bpf_map_ops cpu_map_ops = {
> +	.map_alloc		= cpu_map_alloc,
> +	.map_free		= cpu_map_free,
> +	.map_delete_elem	= cpu_map_delete_elem,
> +	.map_update_elem	= cpu_map_update_elem,
> +	.map_lookup_elem	= cpu_map_lookup_elem,
> +	.map_get_next_key	= cpu_map_get_next_key,
> +};
> +
> +

Extra new line.

> +/* Runs under RCU-read-side, plus in softirq under NAPI protection.
> + * Thus, safe percpu variable access.
> + */
> +static int bq_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_pkt *xdp_pkt)
> +{
> +	struct xdp_bulk_queue *bq = this_cpu_ptr(rcpu->bulkq);
> +
> +	if (unlikely(bq->count == CPU_MAP_BULK_SIZE)) {
> +		bq_flush_to_queue(rcpu, bq);
> +	}

Curly brackets not needed.