linux-kernel - Re: [apparmor] use per-cpu refcounts for apparmor labels?

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Date:   Tue, 26 Sep 2023 05:48:47 -0700
From:   John Johansen <john.johansen@...onical.com>
To:     Mateusz Guzik <mjguzik@...il.com>
Cc:     Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        linux-security-module@...r.kernel.org, apparmor@...ts.ubuntu.com,
        linux-kernel@...r.kernel.org
Subject: Re: [apparmor] use per-cpu refcounts for apparmor labels?

On 9/25/23 23:38, Mateusz Guzik wrote:
> On Mon, Sep 25, 2023 at 11:21:26PM -0700, John Johansen wrote:
>> On 9/25/23 16:49, Vinicius Costa Gomes wrote:
>>> Hi Mateusz,
>>>
>>> Mateusz Guzik <mjguzik@...il.com> writes:
>>>
>>>> I'm sanity-checking perf in various microbenchmarks and I found
>>>> apparmor to be the main bottleneck in some of them.
>>>>
>>>> For example: will-it-scale open1_processes -t 16, top of the profile:
>>>>     20.17%  [kernel]                   [k] apparmor_file_alloc_security
>>>>     20.08%  [kernel]                   [k] apparmor_file_open
>>>>     20.05%  [kernel]                   [k] apparmor_file_free_security
>>>>     18.39%  [kernel]                   [k] apparmor_current_getsecid_subj
>>>> [snip]
>>>>
>>>> This serializes on refing/unrefing apparmor objs, sounds like a great
>>>> candidate for per-cpu refcounting instead (I'm assuming they are
>>>> expected to be long-lived).
>>>>
>>>> I would hack it up myself, but I failed to find a clear spot to switch
>>>> back from per-cpu to centalized operation and don't want to put
>>>> serious effort into it.
>>>>
>>>> Can you sort this out?
>>>
>>
>> I will add looking into it on the todo list. Its going to have to come
>> after some other major cleanups land, and I am not sure we can make
>> the semantic work well for some of these. For other we might get away
>> with switching to a critical section like Vinicius's patch has done
>> for apparmor_current_getsecid_subj.
>>
> 
> Is there an eta?
> 
sorry no

> I looked at dodging ref round trips myself, but then found that ref
> manipulation in apparmor_file_alloc_security and the free counterpart
> cannot be avoided. Thus per-cpu refs instead.
> 

right for file_aloc/free, I don't see a way around keeping a ref count.

> Perhaps making the label as stale would be a good enough switching
> point? Is it *guaranteed* to get labelled as stale before it gets freed?
> 
no. the stale flag only indicates the label has been replaced, and we
make no guarentees as to when it will get set/be in use beyond so
point after it happens.

> btw, __aa_proxy_redirect open-codes setting the flag.
> 
yes, I am aware.

>>> I was looking at this same workload, and proposed a patch[1] some time
>>> ago, see if it helps:
>>>
>>> https://lists.ubuntu.com/archives/apparmor/2023-August/012914.html
>>>
>>> But my idea was different, in many cases, we are looking at the label
>>> associated with the current task, and there's no need to take the
>>> refcount.
>>>
>>
>> yes, and thanks for that.
>>