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#!/usr/bin/env bpftrace
/*
 * tcpsynbl - Show TCP SYN backlog as a histogram.
 *
 * See BPF Performance Tools, Chapter 10, for an explanation of this tool.
 *
 * Copyright (c) 2019 Brendan Gregg.
 * Licensed under the Apache License, Version 2.0 (the "License").
 * This was originally created for the BPF Performance Tools book
 * published by Addison Wesley. ISBN-13: 9780136554820
 * When copying or porting, include this comment.
 *
 * 19-Apr-2019  Brendan Gregg   Created this.
 */

#ifndef BPFTRACE_HAVE_BTF
#include <net/sock.h>
#endif

BEGIN
{
	printf("Tracing SYN backlog size. Ctrl-C to end.\n");
}

kprobe:tcp_v4_syn_recv_sock,
kprobe:tcp_v6_syn_recv_sock
{
	$sock = (struct sock *)arg0;
	@backlog[$sock->sk_max_ack_backlog & 0xffffffff] =
	    hist($sock->sk_ack_backlog);
	if ($sock->sk_ack_backlog > $sock->sk_max_ack_backlog) {
		time("%H:%M:%S dropping a SYN.\n");
	}
}

END
{
	printf("\n@backlog[backlog limit]: histogram of backlog size\n");
}