On Thu, 11 May 2023 at 13:49, Etienne Carriere etienne.carriere@linaro.org wrote:
On Thu, 11 May 2023 at 09:27, Sumit Garg sumit.garg@linaro.org 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.
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?
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.
-Sumit