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From:   Anil Altinay <aaltinay@google.com>
Date:   Mon, 26 Jun 2023 16:35:37 -0700
Message-ID: <CACCxZWNx7=+BvrGgPYTGwARKdJp4DhKvGXAPdzuKu+ebmHr7Xw@mail.gmail.com>
Subject: Re: [PATCH v3] apparmor: global buffers spin lock may get contended
To:     John Johansen <john.johansen@canonical.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        LKLM <linux-kernel@vger.kernel.org>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Tomasz Figa <tfiga@chromium.org>,
        linux-security-module@vger.kernel.org
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Hi John,

I was wondering if you get a chance to work on patch v4. Please let me
know if you need help with testing.

Best,
Anil


On Tue, Feb 21, 2023 at 1:27=E2=80=AFPM Anil Altinay <aaltinay@google.com> =
wrote:
>
> I can test the patch with 5.10 and 5.15 kernels in different machines.
> Just let me know which machine types you would like me to test.
>
> On Mon, Feb 20, 2023 at 12:42 AM John Johansen
> <john.johansen@canonical.com> wrote:
> >
> > On 2/17/23 02:44, Sebastian Andrzej Siewior wrote:
> > > On 2023-02-16 16:08:10 [-0800], John Johansen wrote:
> > >> --- a/security/apparmor/lsm.c
> > >> +++ b/security/apparmor/lsm.c
> > >> @@ -49,12 +49,19 @@ union aa_buffer {
> > >>      char buffer[1];
> > >>   };
> > >> +struct aa_local_cache {
> > >> +    unsigned int contention;
> > >> +    unsigned int hold;
> > >> +    struct list_head head;
> > >> +};
> > >
> > > if you stick a local_lock_t into that struct, then you could replace
> > >       cache =3D get_cpu_ptr(&aa_local_buffers);
> > > with
> > >       local_lock(&aa_local_buffers.lock);
> > >       cache =3D this_cpu_ptr(&aa_local_buffers);
> > >
> > > You would get the preempt_disable() based locking for the per-CPU
> > > variable (as with get_cpu_ptr()) and additionally some lockdep
> > > validation which would warn if it is used outside of task context (IR=
Q).
> > >
> > I did look at local_locks and there was a reason I didn't use them. I
> > can't recall as the original iteration of this is over a year old now.
> > I will have to dig into it again.
> >
> > > I didn't parse completely the hold/contention logic but it seems to w=
ork
> > > ;)
> > > You check "cache->count >=3D  2" twice but I don't see an inc/ dec of=
 it
> > > nor is it part of aa_local_cache.
> > >
> > sadly I messed up the reordering of this and the debug patch. This will=
 be
> > fixed in v4.
> >
> > > I can't parse how many items can end up on the local list if the glob=
al
> > > list is locked. My guess would be more than 2 due the ->hold paramete=
r.
> > >
> > So this iteration, forces pushing back to global list if there are alre=
ady
> > two on the local list. The hold parameter just affects how long the
> > buffers remain on the local list, before trying to place them back on
> > the global list.
> >
> > Originally before the count was added more than 2 buffers could end up
> > on the local list, and having too many local buffers is a waste of
> > memory. The count got added to address this. The value of 2 (which shou=
ld
> > be switched to a define) was chosen because no mediation routine curren=
tly
> > uses more than 2 buffers.
> >
> > Note that this doesn't mean that more than two buffers can be allocated
> > to a tasks on a cpu. Its possible in some cases to have a task have
> > allocated buffers and to still have buffers on the local cache list.
> >
> > > Do you have any numbers on the machine and performance it improved? I=
t
> > > sure will be a good selling point.
> > >
> >
> > I can include some supporting info, for a 16 core machine. But it will
> > take some time to for me to get access to a bigger machine, where this
> > is much more important. Hence the call for some of the other people
> > on this thread to test.
> >
> > thanks for the feedback
> >