In our production system, we observed rcu stalls when
'bpftool prog` is running.
  rcu: INFO: rcu_sched self-detected stall on CPU
  rcu: \x097-....: (20999 ticks this GP) idle=302/1/0x4000000000000000 softirq=1508852/1508852 fqs=4913
  \x09(t=21031 jiffies g=2534773 q=179750)
  NMI backtrace for cpu 7
  CPU: 7 PID: 184195 Comm: bpftool Kdump: loaded Tainted: G        W         5.8.0-00004-g68bfc7f8c1b4 #6
  Hardware name: Quanta Twin Lakes MP/Twin Lakes Passive MP, BIOS F09_3A17 05/03/2019
  Call Trace:
  <IRQ>
  dump_stack+0x57/0x70
  nmi_cpu_backtrace.cold+0x14/0x53
  ? lapic_can_unplug_cpu.cold+0x39/0x39
  nmi_trigger_cpumask_backtrace+0xb7/0xc7
  rcu_dump_cpu_stacks+0xa2/0xd0
  rcu_sched_clock_irq.cold+0x1ff/0x3d9
  ? tick_nohz_handler+0x100/0x100
  update_process_times+0x5b/0x90
  tick_sched_timer+0x5e/0xf0
  __hrtimer_run_queues+0x12a/0x2a0
  hrtimer_interrupt+0x10e/0x280
  __sysvec_apic_timer_interrupt+0x51/0xe0
  asm_call_on_stack+0xf/0x20
  </IRQ>
  sysvec_apic_timer_interrupt+0x6f/0x80
  asm_sysvec_apic_timer_interrupt+0x12/0x20
  RIP: 0010:task_file_seq_get_next+0x71/0x220
  Code: 00 00 8b 53 1c 49 8b 7d 00 89 d6 48 8b 47 20 44 8b 18 41 39 d3 76 75 48 8b 4f 20 8b 01 39 d0 76 61 41 89 d1 49 39 c1 48 19 c0 <48> 8b 49 08 21 d0 48 8d 04 c1 4c 8b 08 4d 85 c9 74 46 49 8b 41 38
  RSP: 0018:ffffc90006223e10 EFLAGS: 00000297
  RAX: ffffffffffffffff RBX: ffff888f0d172388 RCX: ffff888c8c07c1c0
  RDX: 00000000000f017b RSI: 00000000000f017b RDI: ffff888c254702c0
  RBP: ffffc90006223e68 R08: ffff888be2a1c140 R09: 00000000000f017b
  R10: 0000000000000002 R11: 0000000000100000 R12: ffff888f23c24118
  R13: ffffc90006223e60 R14: ffffffff828509a0 R15: 00000000ffffffff
  task_file_seq_next+0x52/0xa0
  bpf_seq_read+0xb9/0x320
  vfs_read+0x9d/0x180
  ksys_read+0x5f/0xe0
  do_syscall_64+0x38/0x60
  entry_SYSCALL_64_after_hwframe+0x44/0xa9
  RIP: 0033:0x7f8815f4f76e
  Code: c0 e9 f6 fe ff ff 55 48 8d 3d 76 70 0a 00 48 89 e5 e8 36 06 02 00 66 0f 1f 44 00 00 64 8b 04 25 18 00 00 00 85 c0 75 14 0f 05 <48> 3d 00 f0 ff ff 77 52 c3 66 0f 1f 84 00 00 00 00 00 55 48 89 e5
  RSP: 002b:00007fff8f9df578 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
  RAX: ffffffffffffffda RBX: 000000000170b9c0 RCX: 00007f8815f4f76e
  RDX: 0000000000001000 RSI: 00007fff8f9df5b0 RDI: 0000000000000007
  RBP: 00007fff8f9e05f0 R08: 0000000000000049 R09: 0000000000000010
  R10: 00007f881601fa40 R11: 0000000000000246 R12: 00007fff8f9e05a8
  R13: 00007fff8f9e05a8 R14: 0000000001917f90 R15: 000000000000e22e

Note that `bpftool prog` actually calls a task_file bpf iterator
program to establish an association between prog/map/link/btf anon
files and processes.

In the case where the above rcu stall occured, we had a process
having 1587 tasks and each task having roughly 81305 files.
This implied 129 million bpf prog invocations. Unfortunwtely none of
these files are prog/map/link/btf files so bpf iterator/prog needs
to traverse all these files and not able to return to user space
since there are no seq_file buffer overflow.

This patch fixed the issue in bpf_seq_read() to limit the number
of visited objects. If the maximum number of visited objects is
reached, no more objects will be visited in the current syscall.
If there is nothing written in the seq_file buffer, -EAGAIN will
return to the user so user can try again.

The maximum number of visited objects is set at 1 million.
In our Intel Xeon D-2191 2.3GHZ 18-core server, bpf_seq_read()
visiting 1 million files takes around 0.18 seconds.

We did not use cond_resched() since for some iterators, e.g.,
netlink iterator, where rcu read_lock critical section spans between
consecutive seq_ops->next(), which makes impossible to do cond_resched()
in the key while loop of function bpf_seq_read().

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/bpf/20200818222309.2181348-1-yhs@fb.com