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. The fix is to add cond_resched() during traversing tasks and files. So voluntarily releasing cpu gives other tasks, e.g., rcu resched kthread, a chance to run. Cc: Paul E. McKenney <paulmck@xxxxxxxxxx> Signed-off-by: Yonghong Song <yhs@xxxxxx> --- kernel/bpf/task_iter.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c index f21b5e1e4540..885b14cab2c0 100644 --- a/kernel/bpf/task_iter.c +++ b/kernel/bpf/task_iter.c @@ -27,6 +27,8 @@ static struct task_struct *task_seq_get_next(struct pid_namespace *ns, struct task_struct *task = NULL; struct pid *pid; + cond_resched(); + rcu_read_lock(); retry: pid = idr_get_next(&ns->idr, tid); @@ -137,6 +139,8 @@ task_file_seq_get_next(struct bpf_iter_seq_task_file_info *info, struct task_struct *curr_task; int curr_fd = info->fd; + cond_resched(); + /* If this function returns a non-NULL file object, * it held a reference to the task/files_struct/file. * Otherwise, it does not hold any reference. -- 2.24.1
