[PATCH v3] apparmor: global buffers spin lock may get contended

[John Johansen]

On 6/26/23 16:33, Anil Altinay wrote:
> 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.
> 

yeah, testing help is always much appreciated. I have a v4, and I am working on 3 alternate version to compare against, to help give a better sense if we can get away with simplifying or tweak the scaling. I should be able to post them out some time tonight.

> Best,
> Anil
> 
> On Tue, Feb 21, 2023 at 1:27 PM Anil Altinay <aaltinay at google.com <mailto:aaltinay at 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 at canonical.com <mailto:john.johansen at 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 = get_cpu_ptr(&aa_local_buffers);
>      > > with
>      > >       local_lock(&aa_local_buffers.lock);
>      > >       cache = 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 (IRQ).
>      > >
>      > 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 work
>      > > ;)
>      > > You check "cache->count >=  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 global
>      > > list is locked. My guess would be more than 2 due the ->hold parameter.
>      > >
>      > So this iteration, forces pushing back to global list if there are already
>      > 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 should
>      > be switched to a define) was chosen because no mediation routine currently
>      > 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? It
>      > > 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
>      >
>