[PATCH v2] drm/xe/client: Better correlate exec_queue and GT timestamps

[Lucas De Marchi]

This partially reverts commit fe4f5d4b6616 ("drm/xe: Clean up VM / exec
queue file lock usage."). While it's desired to have the mutex to
protect only the reference to the exec queue, getting and dropping each
mutex and then later getting the GPU timestamp, doesn't produce a
correct result: it introduces multiple opportunities for the task to be
scheduled out and thus wrecking havoc the deltas reported to userspace.

Also, to better correlate the timestamp from the exec queues with the
GPU, disable preemption so they can be updated without allowing the task
to be scheduled out. We leave interrupts enabled as that shouldn't be
enough disturbance for the deltas to matter to userspace.

Test scenario:

	* IGT'S `xe_drm_fdinfo --r utilization-single-full-load`
	* Platform: LNL, where CI occasionally reports failures
	* `stress -c $(nproc)` running in parallel to disturb the
	  system

This brings a first failure from "after ~150 executions" to "never
occurs after 1000 attempts".

v2: Also keep xe_hw_engine_read_timestamp() call inside the
    preemption-disabled section (Umesh)

Cc: stable at vger.kernel.org # v6.11+
Cc: Umesh Nerlige Ramappa <umesh.nerlige.ramappa at intel.com>
Cc: Matthew Brost <matthew.brost at intel.com>
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/3512
Signed-off-by: Lucas De Marchi <lucas.demarchi at intel.com>
---
 drivers/gpu/drm/xe/xe_drm_client.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_drm_client.c b/drivers/gpu/drm/xe/xe_drm_client.c
index 7d55ad846bac5..2220a09bf9751 100644
--- a/drivers/gpu/drm/xe/xe_drm_client.c
+++ b/drivers/gpu/drm/xe/xe_drm_client.c
@@ -337,20 +337,18 @@ static void show_run_ticks(struct drm_printer *p, struct drm_file *file)
 		return;
 	}
 
+	/* Let both the GPU timestamp and exec queue be updated together */
+	preempt_disable();
+	gpu_timestamp = xe_hw_engine_read_timestamp(hwe);
+
 	/* Accumulate all the exec queues from this client */
 	mutex_lock(&xef->exec_queue.lock);
-	xa_for_each(&xef->exec_queue.xa, i, q) {
-		xe_exec_queue_get(q);
-		mutex_unlock(&xef->exec_queue.lock);
 
+	xa_for_each(&xef->exec_queue.xa, i, q)
 		xe_exec_queue_update_run_ticks(q);
 
-		mutex_lock(&xef->exec_queue.lock);
-		xe_exec_queue_put(q);
-	}
 	mutex_unlock(&xef->exec_queue.lock);
-
-	gpu_timestamp = xe_hw_engine_read_timestamp(hwe);
+	preempt_enable();
 
 	xe_force_wake_put(gt_to_fw(hwe->gt), fw_ref);
 	xe_pm_runtime_put(xe);
-- 
2.47.0