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Message-ID: <dd3f0a9f-ec3c-46c3-8231-2022b67094a6@huawei.com>
Date: Tue, 3 Dec 2024 21:30:09 +0800
From: Kefeng Wang <wangkefeng.wang@...wei.com>
To: zuoze <zuoze1@...wei.com>, Uladzislau Rezki <urezki@...il.com>
CC: Matthew Wilcox <willy@...radead.org>, <gustavoars@...nel.org>,
<akpm@...ux-foundation.org>, <linux-hardening@...r.kernel.org>,
<linux-mm@...ck.org>, <keescook@...omium.org>
Subject: Re: [PATCH -next] mm: usercopy: add a debugfs interface to bypass the
vmalloc check.
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.
>>>
>> 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@...il.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.
>
It is unclear, we have backported the vmalloc optimization into our 6.6
kernel before, so the above stack already with those patches and even
with those optimization, the find_vmap_area() is still the hotpots.
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