app: Add work queue for long blocking operations

[MarkusLassila]

We don't want to block the AT-command processing for long periods of time when we do long blocking operations.

• Add a separate work queue for those.
• Drop the sm_work_q stack size to 2048 as the blocking operations are the ones with most stack usage.

[Copilot]

Pull request overview

This PR introduces a dedicated Zephyr work queue intended for long/blocking operations so the Serial Modem’s primary AT-command processing path (driven via the existing sm_work_q) is less likely to be stalled by lengthy tasks (e.g., nRF Cloud connect/location requests).

Changes:

• Adds a new optional sm_blocking_work_q work queue (with configurable stack size) alongside the existing sm_work_q.
• Starts the new blocking work-queue thread from main() when enabled.
• Moves several nRF Cloud work items from sm_work_q to sm_blocking_work_q and adds busy-guarding for connect/disconnect work.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
app/src/sm_util.h	Exposes sm_blocking_work_q (when enabled) as a shared extern.
app/src/sm_at_nrfcloud.c	Routes nRF Cloud connect/disconnect and location request work to the blocking queue; adds k_work_busy_get() checks.
app/src/main.c	Defines, initializes, and starts the blocking work queue thread.
app/Kconfig	Adds SM_BLOCKING_WORK_Q and stack-size config; enables it for SM_NRF_CLOUD.

Suppressed comments (4)

app/src/sm_at_nrfcloud.c:257

• This work is submitted to sm_blocking_work_q without guarding CONFIG_SM_BLOCKING_WORK_Q. This breaks the at_nrfcloud unit test build (no CONFIG_SM_BLOCKING_WORK_Q and no sm_blocking_work_q stub) and makes the module harder to reuse in configurations that don’t enable the blocking queue. Add a compile-time fallback to sm_work_q.

			k_work_submit_to_queue(&sm_blocking_work_q, &nrfcloud_conn_work);

app/src/sm_at_nrfcloud.c:456

• This submits the location request work to sm_blocking_work_q unconditionally. The at_nrfcloud unit test build compiles this file without CONFIG_SM_BLOCKING_WORK_Q (and only provides sm_work_q), so this will fail to compile there. Consider a compile-time fallback to sm_work_q when the blocking queue isn’t enabled.

		k_work_submit_to_queue(&sm_blocking_work_q, &nrfcloud_loc_req_work);

app/src/sm_at_nrfcloud.c:589

• This submits nrfcloud_loc_req_work to sm_blocking_work_q unconditionally. In the at_nrfcloud unit test build CONFIG_SM_BLOCKING_WORK_Q isn’t defined, so sm_blocking_work_q isn’t declared/defined and this will fail to compile. Add a compile-time fallback to sm_work_q when the blocking queue is unavailable.

		k_work_submit_to_queue(&sm_blocking_work_q, &nrfcloud_loc_req_work);

app/src/sm_at_nrfcloud.c:622

• This submits nrfcloud_loc_req_work to sm_blocking_work_q unconditionally. The at_nrfcloud unit test target doesn’t define CONFIG_SM_BLOCKING_WORK_Q and only stubs sm_work_q, so this breaks that build. Add a compile-time fallback to sm_work_q when the blocking queue isn’t enabled.

			k_work_submit_to_queue(&sm_blocking_work_q, &nrfcloud_loc_req_work);

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[MarkusLassila]

Cannot test locally. Running the integration tests in CI.

[github-actions]

Memory usage changed for nRF91M1 configuration. See CI run

       text       data        bss        dec        hex     flash%       ram%
--- baseline
+++ pr
@@ -2 +2 @@
-    319456      74588     167246     561290      8908a     95.29%     82.07%
+    319540      74616     169534     563690      899ea     95.31%     83.15%

[trantanen]

As discussed, we can decrease SM work queue stack size from 4kB to 2kB when nRF Cloud location requests are not there anymore. Those requests require ~3.3kB of stack due to nrf_cloud_coap library implementation.

[MarkusLassila]

Done

[trantanen]

If we are merging this PR, I'm not sure we should have this configurability for the blocking work queue.

[MarkusLassila]

I want this off for Native TLS. The nrf cloud does not fit in there and RAM is hard to grab.