netdev - Re: Very slow remove interface from kernel

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Date: Tue, 9 May 2023 23:08:20 +0300
From: Ido Schimmel <idosch@...sch.org>
To: Martin Zaharinov <micron10@...il.com>
Cc: Eric Dumazet <edumazet@...gle.com>, netdev <netdev@...r.kernel.org>
Subject: Re: Very slow remove interface from kernel

On Tue, May 09, 2023 at 09:50:18PM +0300, Martin Zaharinov wrote:
> i try on kernel 6.3.1 
> 
> 
> time for i in $(seq 2 4094); do ip link del link eth1 name vlan$i type vlan id $i; done
> 
> real	4m51.633s  —— here i stop with Ctrl + C  -  and rerun  and second part finish after 3 min
> user	0m7.479s
> sys	0m0.367s

You are off-CPU most of the time, the question is what is blocking. I'm
getting the following results with net-next:

# time -p for i in $(seq 2 4094); do ip link del dev eth0.$i; done
real 177.09
user 3.85
sys 31.26

When using a batch file to perform the deletion:

# time -p ip -b vlan_del.batch 
real 35.25
user 0.02
sys 3.61

And to check where we are blocked most of the time while using the batch
file:

# ../bcc/libbpf-tools/offcputime -p `pgrep -nx ip`
[...]
    __schedule
    schedule
    schedule_timeout
    wait_for_completion
    rcu_barrier
    netdev_run_todo
    rtnetlink_rcv_msg
    netlink_rcv_skb
    netlink_unicast
    netlink_sendmsg
    ____sys_sendmsg
    ___sys_sendmsg
    __sys_sendmsg
    do_syscall_64
    entry_SYSCALL_64_after_hwframe
    -                ip (3660)
        25089479
[...]

We are blocked for around 70% of the time on the rcu_barrier() in
netdev_run_todo().

Note that one big difference between my setup and yours is that in my
case eth0 is a dummy device and in your case it's probably a physical
device that actually implements netdev_ops::ndo_vlan_rx_kill_vid(). If
so, it's possible that a non-negligible amount of time is spent talking
to hardware/firmware to delete the 4K VIDs from the device's VLAN
filter.

> 
> 
> Config is very clean i remove big part of CONFIG options .
> 
> is there options to debug what is happen.
> 
> m