On Fri, 27 Sep 2019, Andrii Nakryiko wrote:
> This patch switches libbpf_num_possible_cpus() from using possible CPU
> set to present CPU set. This fixes issues with incorrect auto-sizing of
> PERF_EVENT_ARRAY map on HOTPLUG-enabled systems.
>
> On HOTPLUG enabled systems, /sys/devices/system/cpu/possible is going to
> be a set of any representable (i.e., potentially possible) CPU, which is
> normally way higher than real amount of CPUs (e.g., 0-127 on VM I've
> tested on, while there were just two CPU cores actually present).
> /sys/devices/system/cpu/present, on the other hand, will only contain
> CPUs that are physically present in the system (even if not online yet),
> which is what we really want, especially when creating per-CPU maps or
> perf events.
>
> On systems with HOTPLUG disabled, present and possible are identical, so
> there is no change of behavior there.
>
Just curious - is there a reason for not adding a new libbpf_num_present_cpus()
function to cover this case, and switching to using that in various places?
Looking at the places libbpf_num_possible_cpus() is called in libbpf
- __perf_buffer__new(): this could just change to use the number of
present CPUs, since perf_buffer__new_raw() with a cpu_cnt in struct
perf_buffer_raw_ops
- bpf_object__create_maps(), which is called via bpf_oject__load_xattr().
In this case it seems like switching to num present makes sense, though
it might make sense to add a field to struct bpf_object_load_attr * to
allow users to explicitly set another max value.
This would give the desired default behaviour, while still giving users
a way of specifying the possible number. What do you think? Thanks!
Alan
> Signed-off-by: Andrii Nakryiko <andriin@xxxxxx>
> ---
> tools/lib/bpf/libbpf.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index e0276520171b..45351c074e45 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -5899,7 +5899,7 @@ void bpf_program__bpil_offs_to_addr(struct bpf_prog_info_linear *info_linear)
>
> int libbpf_num_possible_cpus(void)
> {
> - static const char *fcpu = "/sys/devices/system/cpu/possible";
> + static const char *fcpu = "/sys/devices/system/cpu/present";
> int len = 0, n = 0, il = 0, ir = 0;
> unsigned int start = 0, end = 0;
> int tmp_cpus = 0;
> --
> 2.17.1
>
>
