Re: [PATCH -next] mm: usercopy: add a debugfs interface to bypass the vmalloc check.

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Dec 03, 2024 at 07:23:44PM +0800, zuoze wrote:
> We have implemented host-guest communication based on the TUN device
> using XSK[1]. The hardware is a Kunpeng 920 machine (ARM architecture),
> and the operating system is based on the 6.6 LTS version with kernel
> version 6.6. The specific stack for hotspot collection is as follows:
> 
> -  100.00%     0.00%  vhost-12384  [unknown]      [k] 0000000000000000
>    - ret_from_fork
>       - 99.99% vhost_task_fn
>          - 99.98% 0xffffdc59f619876c
>             - 98.99% handle_rx_kick
>                - 98.94% handle_rx
>                   - 94.92% tun_recvmsg
>                      - 94.76% tun_do_read
>                         - 94.62% tun_put_user_xdp_zc
>                            - 63.53% __check_object_size
>                               - 63.49% __check_object_size.part.0
>                                    find_vmap_area
>                            - 30.02% _copy_to_iter
>                                 __arch_copy_to_user
>                   - 2.27% get_rx_bufs
>                      - 2.12% vhost_get_vq_desc
>                           1.49% __arch_copy_from_user
>                   - 0.89% peek_head_len
>                        0.54% xsk_tx_peek_desc
>                   - 0.68% vhost_add_used_and_signal_n
>                      - 0.53% eventfd_signal
>                           eventfd_signal_mask
>             - 0.94% handle_tx_kick
>                - 0.94% handle_tx
>                   - handle_tx_copy
>                      - 0.59% vhost_tx_batch.constprop.0
>                           0.52% tun_sendmsg
> 
> It can be observed that most of the overhead is concentrated in the
> find_vmap_area function.
> 
I see. Yes, it is pretty contented, since you run the v6.6 kernel. There
was a work that tends to improve it to mitigate a vmap lock contention.
See it here: https://lwn.net/Articles/956590/

The work was taken in the v6.9 kernel:

<snip>
commit 38f6b9af04c4b79f81b3c2a0f76d1de94b78d7bc
Author: Uladzislau Rezki (Sony) <urezki@gmail.com>
Date:   Tue Jan 2 19:46:23 2024 +0100

    mm: vmalloc: add va_alloc() helper

    Patch series "Mitigate a vmap lock contention", v3.

    1. Motivation
...
<snip>

Could you please try the v6.9 kernel on your setup?

How to solve it, probably, it can be back-ported to the v6.6 kernel. 

Thanks!

--
Uladzislau Rezki