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Date:   Wed, 13 Apr 2022 22:06:29 +0800
From:   Wu Zongyong <wuzongyong@linux.alibaba.com>
To:     bpf@vger.kernel.org
Subject: [Question] bpf map value increase unexpectedly with tracepoint
 qdisc/qdisc_dequeue
Message-ID: <20220413140629.GA22650@L-PF27918B-1352.localdomain>
Reply-To: Wu Zongyong <wuzongyong@linux.alibaba.com>
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Hi,

I tried to count when tracepoint qdisc/qdisc_dequeue hit each time, then read
the count value from userspace by bpf_map_lookup_elem().
With bpftrace, I can see this tracepoint is hit about 700 times, but the count
I get from the bpf map is below 20. It's weird. Then I tried to add a bpf_printk()
to the program, and the count I get is normal which is about 700.

The bpf program codes like that:

	struct qdisc_dequeue_ctx {
	        __u64           __pad;
	        __u64           qdisc;
	        __u64           txq;
	        int             packets;
	        __u64           skbaddr;
	        int             ifindex;
	        __u32           handle;
	        __u32           parent;
	        unsigned long   txq_state;
	};
	
	struct {
	        __uint(type, BPF_MAP_TYPE_HASH);
	        __type(key, int);
	        __type(value, __u32);
	        __uint(max_entries, 1);
	        __uint(pinning, LIBBPF_PIN_BY_NAME);
	} count_map SEC(".maps");
	
	SEC("tracepoint/qdisc/qdisc_dequeue")
	int trace_dequeue(struct qdisc_dequeue_ctx *ctx)
	{
	        int key = 0;
	        __u32 *value;
	        __u32 init = 0;
	
	        value = bpf_map_lookup_elem(&count_map, &key);
	        if (value) {
	                *value += 1;
	        } else {
	                bpf_printk("value reset");
	                bpf_map_update_elem(&count_map, &key, &init, 0);
	        }
	        return 0;
	}

Any suggestion is appreciated!

Thanks,
Wu Zongyong