linux-kernel - Re: [PATCH 2/3] audit: annotate branch direction for audit_in

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Date:   Thu, 06 Oct 2022 17:57:26 -0700
From:   Ankur Arora <ankur.a.arora@...cle.com>
To:     Paul Moore <paul@...l-moore.com>
Cc:     Ankur Arora <ankur.a.arora@...cle.com>, linux-audit@...hat.com,
        eparis@...hat.com, linux-kernel@...r.kernel.org,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH 2/3] audit: annotate branch direction for audit_in_mask()


Paul Moore <paul@...l-moore.com> writes:

> On Thu, Sep 29, 2022 at 4:20 PM Ankur Arora <ankur.a.arora@...cle.com> wrote:
>> Paul Moore <paul@...l-moore.com> writes:
>> > I generally dislike merging likely()/unlikely() additions to code
>> > paths that can have varying levels of performance depending on runtime
>> > configuration.
>>
>> I think that's fair, and in this particular case the benchmark is quite
>> contrived.
>>
>> But, just to elaborate a bit more on why that unlikely() clause made
>> sense to me: it seems to me that audit typically would be triggered for
>> control syscalls and the ratio between control and non-control ones
>> would be fairly lopsided.
>
> I understand, and there is definitely some precedence in the audit
> code for using likely()/unlikely() in a manner similar as you
> described, but I'll refer to my previous comments - it's not something
> I like.  As a general rule, aside from the unlikely() calls in the
> audit wrappers present in include/linux/audit.h I would suggest not
> adding any new likely()/unlikely() calls.
>
>> Let me see if I can rewrite the conditional in a different way to get a
>> similar effect but I suspect that might be even more compiler dependent.
>
> I am okay with ordering conditionals to make the common case the
> short-circuit case.

So I played around with a bunch of different combinations of the
conditionals but nothing really improved the code all that much.

Just sent out v2 dropping the unlikely() clause.


Thanks
Ankur

>
>> Also, let me run the audit-testsuite this time. Is there a good test
>> there that you would recommend that might serve as a more representative
>> workload?
>
> Probably not.  The audit-testsuite is intended to be a quick, easy to
> run regression test that can be used by developers to help reduce
> audit breakage.  It is not representative of any particular workload,
> and is definitely not comprehensive (it is woefully lacking in several
> areas unfortunately).


--
ankur