Re: [PATCH v6 3/4] tee: optee: support tracking system threads

[Etienne Carriere]

On Thu, 11 May 2023 at 13:31, Sumit Garg <sumit.garg@xxxxxxxxxx> wrote:
>
> On Thu, 11 May 2023 at 13:49, Etienne Carriere
> <etienne.carriere@xxxxxxxxxx> wrote:
> >
> > On Thu, 11 May 2023 at 09:27, Sumit Garg <sumit.garg@xxxxxxxxxx> wrote:
> > > (snip)
> > > > > > >
> > > > > > > +bool optee_cq_inc_sys_thread_count(struct optee_call_queue *cq)
> > > > > > > +{
> > > > > > > +       bool rc = false;
> > > > > > > +
> > > > > > > +       mutex_lock(&cq->mutex);
> > > > > > > +
> > > > > > > +       /* Leave at least 1 normal (non-system) thread */
> > > > > >
> > > > > > IMO, this might be counter productive. As most kernel drivers open a
> > > > > > session during driver probe which are only released in the driver
> > > > > > release method.
> > > > >
> > > > > It is always the case?
> > > >
> > > > This answer of mine is irrelevant. Sorry,
> > > > Please read only the below comments of mine, especially:
> > > > | Note that an OP-TEE thread is not bound to a TEE session but rather
> > > > | bound to a yielded call to OP-TEE.
> > > >
> > > > >
> > > > > > If the kernel driver is built-in then the session is
> > > > > > never released. Now with system threads we would reserve an OP-TEE
> > > > > > thread for that kernel driver as well which will never be available to
> > > > > > regular user-space clients.
> > > > >
> > > > > That is not true. No driver currently requests their TEE thread to be
> > > > > a system thread.
> > > > > Only SCMI does because it needs to by construction.
> > > > >
> > >
> > > Yes that's true but what prevents future/current kernel TEE drivers
> > > from requesting a system thread once we have this patch-set landed.
> >
> > Only clients really needing this system_thread attribute should request it.
> > If they really need, the OP-TEE firmware in secure world should
> > provision sufficient thread context.
>
> How do we quantify it? We definitely need a policy here regarding
> normal vs system threads.
>
> One argument in favor of kernel clients requiring system threads could
> be that we don't want to compete with user-space for OP-TEE threads.

Sorry I don't understand. What do you mean qualifying this?
In an ideal situation, we would have OP-TEE provisioned with largely
sufficient thread contexts. However there are systems with constraints
memory resource that do lower at most the number of OP-TEE thread
contexts.



>
> >
> > >
> > > > >
> > > > > > So I would rather suggest we only allow a
> > > > > > single system thread to be reserved as a starting point which is
> > > > > > relevant to this critical SCMI service. We can also make this upper
> > > > > > bound for system threads configurable with default value as 1 if
> > > > > > needed.
> > > >
> > > > Note that SCMI server can expose several SCMI channels (at most 1 per
> > > > SCMI protocol used) and each of them will need to request a
> > > > system_thread to TEE driver.
> > > >
> > > > Etienne
> > > >
> > > > >
> > > > > Reserving one or more system threads depends on the number of thread
> > > > > context provisioned by the TEE.
> > > > > Note that the implementation proposed here prevents Linux kernel from
> > > > > exhausting TEE threads so user space always has at least a TEE thread
> > > > > context left available.
> > >
> > > Yeah but on the other hand user-space clients which are comparatively
> > > larger in number than kernel clients. So they will be starved for
> > > OP-TEE thread availability. Consider a user-space client which needs
> > > to serve a lot of TLS connections just waiting for OP-TEE thread
> > > availability.
> >
> > Note that OP-TEE default configuration provisions (number of CPUs + 1)
> > thread context, so the situation is already present before these
> > changes on systems that embedded an OP-TEE without a properly tuned
> > configuration. As I said above, Linux kernel cannot be responsible for
> > the total number of thread contexts provisioned in OP-TEE. If the
> > overall system requires a lot of TEE thread contexts, one should embed
> > a suitable OP-TEE firmware.
>
> Wouldn't the SCMI deadlock problem be solved with just having a lot of
> OP-TEE threads? But we are discussing the system threads solution here
> to make efficient use of OP-TEE threads. The total number of OP-TEE
> threads is definitely in control of OP-TEE but the control of how to
> schedule and efficiently use them lies with the Linux OP-TEE driver.
>
> So, given our overall discussion in this thread, how about the upper
> bound for system threads being 50% of the total number of OP-TEE
> threads?

What would be a shame if the system does not use any Linux kernel
client sessions, only userland clients. This information cannot be
knwon be the linux optee driver.
Instead of leaving at least 1 TEE thread context for regular session,
what if this change enforce 2? or 3? Which count?
I think 1 is a fair choice: it allows to support OP-TEE firmwares with
a very small thread context pool (when running in small secure
memory), embedding only 2 or 3 contextes.

>
> >
> > >
> > > > >
> > > > > Note that an OP-TEE thread is not bound to a TEE session but rather
> > > > > bound to a yielded call to OP-TEE.
> > >
> > > tee_client_open_session()
> > >   -> optee_open_session()
> > >
> > > tee_client_system_session()
> > >   -> optee_system_session()
> > >     -> optee_cq_inc_sys_thread_count()       <- At this point you
> > > reserve a system thread corresponding to a particular kernel client
> > > session
> > >
> > > All tee_client_invoke_func() invocations with a system thread capable
> > > session will use that reserved thread.
> > >
> > > tee_client_close_session()
> > >   -> optee_close_session()
> > >     -> optee_close_session_helper()
> > >       -> optee_cq_dec_sys_thread_count()    <- At this point the
> > > reserved system thread is released
> > >
> > > Haven't this tied the system thread to a particular TEE session? Or am
> > > I missing something?
> >
> > These changes do not define an overall single system thread.
> > If several sessions requests reservation of TEE system thread, has
> > many will be reserved.
> > Only the very sessions with its sys_thread attribute set will use a
> > reserved thread. If such a kernel client issues several concurrent
> > calls to OP-TEE over that session, it will indeed consume more
> > reserved system threads than what is actually reserved. Here I think
> > it is the responsibility of such client to open as many sessions as
> > requests. This is what scmi/optee driver does (see patch v6 4/4). An
> > alternative would be to have a ref count of sys_thread in session
> > contexts rather than a boolean value. I don't think it's worth it.
>
> Ah, I missed that during the review. The invocations with system
> threads should be limited by res_sys_thread_count in a similar manner
> as we do with normal threads via free_normal_thread_count. Otherwise,
> it's unfair for normal thread scheduling.
>
> I suppose there isn't any interdependency among SCMI channels itself
> such that a particular SCMI invocation can wait until the other SCMI
> invocation has completed.

I think that would over complexify the logic.

Note I will send a patch v8 series but feel free to continue the discussion.
It will at least address other comments you shared.

Best regards,
Etienne

>
> -Sumit