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

[zuoze <zuoze1@huawei.com>]

在 2024/12/3 20:39, Uladzislau Rezki 写道:
> 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.

All the vmalloc-related optimizations have already been merged into 6.6,
including the set of optimization patches you suggested. Thank you very
much for your input.

> 
> Thanks!
> 
> --
> Uladzislau Rezki
> 

Thanks,
ze zuo