netdev - [PATCH bpf] libbpf: count present CPUs, not theoretically possible

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Date:   Fri, 27 Sep 2019 23:30:33 -0700
From:   Andrii Nakryiko <andriin@...com>
To:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>, <ast@...com>,
        <daniel@...earbox.net>
CC:     <andrii.nakryiko@...il.com>, <kernel-team@...com>,
        Andrii Nakryiko <andriin@...com>
Subject: [PATCH bpf] libbpf: count present CPUs, not theoretically possible

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.

Signed-off-by: Andrii Nakryiko <andriin@...com>
---
 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