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 10:10:26PM +0800, Kefeng Wang wrote:
> 
> 
> On 2024/12/3 21:51, Uladzislau Rezki wrote:
> > On Tue, Dec 03, 2024 at 09:45:09PM +0800, Kefeng Wang wrote:
> > > 
> > > 
> > > On 2024/12/3 21:39, Uladzislau Rezki wrote:
> > > > On Tue, Dec 03, 2024 at 09:30:09PM +0800, Kefeng Wang wrote:
> > > > > 
> > > > > 
> > > > > On 2024/12/3 21:10, zuoze wrote:
> > > > > > 
> > > > > > 
> > > > > > 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.
> > > > > > > > 
> ...
> > > 
> > Thank you. Then you have tons of copy_to_iter/copy_from_iter calls
> > during your test case. Per each you need to find an area which might
> > be really heavy.
> 
> Exactly, no vmalloc check before 0aef499f3172 （"mm/usercopy: Detect vmalloc
> overruns"), so no burden in find_vmap_area in old kernel.
> 
Yep. It will slow down for sure.

> > 
> > How many CPUs in a system you have?
> > 
> 
> 128 core
OK. Just in case, do you see in a boot log something like:

"Failed to allocate an array. Disable a node layer"

Thanks!

--
Uladzislau Rezki