netdev - Re: [PATCH v2 bpf-next 2/3] libbpf: auto-set PERF_EVENT

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Date:   Wed, 26 Jun 2019 18:57:49 +0000
From:   Song Liu <songliubraving@...com>
To:     Andrii Nakryiko <andriin@...com>
CC:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <ast@...com>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 2/3] libbpf: auto-set PERF_EVENT_ARRAY size to
 number of CPUs



> On Jun 25, 2019, at 11:12 PM, Andrii Nakryiko <andriin@...com> wrote:
> 
> For BPF_MAP_TYPE_PERF_EVENT_ARRAY typically correct size is number of
> possible CPUs. This is impossible to specify at compilation time. This
> change adds automatic setting of PERF_EVENT_ARRAY size to number of
> system CPUs, unless non-zero size is specified explicitly. This allows
> to adjust size for advanced specific cases, while providing convenient
> and logical defaults.
> 
> Signed-off-by: Andrii Nakryiko <andriin@...com>
> ---
> tools/lib/bpf/libbpf.c | 17 ++++++++++++++++-
> 1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index c74cc535902a..8f2b8a081ba7 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -2114,6 +2114,7 @@ static int
> bpf_object__create_maps(struct bpf_object *obj)
> {
> 	struct bpf_create_map_attr create_attr = {};
> +	int nr_cpus = 0;
> 	unsigned int i;
> 	int err;
> 
> @@ -2136,7 +2137,21 @@ bpf_object__create_maps(struct bpf_object *obj)
> 		create_attr.map_flags = def->map_flags;
> 		create_attr.key_size = def->key_size;
> 		create_attr.value_size = def->value_size;
> -		create_attr.max_entries = def->max_entries;
> +		if (def->type == BPF_MAP_TYPE_PERF_EVENT_ARRAY &&
> +		    !def->max_entries) {
> +			if (!nr_cpus)
> +				nr_cpus = libbpf_num_possible_cpus();
> +			if (nr_cpus < 0) {
> +				pr_warning("failed to determine number of system CPUs: %d\n",
> +					   nr_cpus);
> +				return nr_cpus;

I think we need to goto err_out here. 

Thanks,
Song