drm/msm: Add VM_BIND throttling

A large number of (unsorted or separate) small (<2MB) mappings can cause
a lot of, probably unnecessary, prealloc pages.  Ie. a single 4k page
size mapping will pre-allocate 3 pages (for levels 2-4) for the
pagetable.  Which can chew up a large amount of unneeded memory.  So add
a mechanism to put an upper bound on the # of pre-alloc pages.

Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>
Tested-by: Antonino Maniscalco <antomani103@gmail.com>
Reviewed-by: Antonino Maniscalco <antomani103@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/661529/

drivers/gpu/drm/msm/msm_gem.h

@@ -75,6 +75,26 @@ struct msm_gem_vm {
 	 */
 	struct drm_gpu_scheduler sched;
 
+	/**
+	 * @prealloc_throttle: Used to throttle VM_BIND ops if too much pre-
+	 * allocated memory is in flight.
+	 *
+	 * Because we have to pre-allocate pgtable pages for the worst case
+	 * (ie. new mappings do not share any PTEs with existing mappings)
+	 * we could end up consuming a lot of resources transiently.  The
+	 * prealloc_throttle puts an upper bound on that.
+	 */
+	struct {
+		/** @wait: Notified when preallocated resources are released */
+		wait_queue_head_t wait;
+
+		/**
+		 * @in_flight: The # of preallocated pgtable pages in-flight
+		 * for queued VM_BIND jobs.
+		 */
+		atomic_t in_flight;
+	} prealloc_throttle;
+
 	/**
 	 * @mm: Memory management for kernel managed VA allocations
 	 *

drivers/gpu/drm/msm/msm_gem_vma.c

@@ -705,6 +705,8 @@ msm_vma_job_free(struct drm_sched_job *_job)
 
 	vm->mmu->funcs->prealloc_cleanup(vm->mmu, &job->prealloc);
 
+	atomic_sub(job->prealloc.count, &vm->prealloc_throttle.in_flight);
+
 	drm_sched_job_cleanup(_job);
 
 	job_foreach_bo (obj, job)
@@ -721,6 +723,8 @@ msm_vma_job_free(struct drm_sched_job *_job)
 		kfree(op);
 	}
 
+	wake_up(&vm->prealloc_throttle.wait);
+
 	kfree(job);
 }
 
@@ -783,6 +787,8 @@ msm_gem_vm_create(struct drm_device *drm, struct msm_mmu *mmu, const char *name,
 		ret = drm_sched_init(&vm->sched, &args);
 		if (ret)
 			goto err_free_dummy;
+
+		init_waitqueue_head(&vm->prealloc_throttle.wait);
 	}
 
 	drm_gpuvm_init(&vm->base, name, flags, drm, dummy_gem,
@@ -1090,10 +1096,12 @@ ops_are_same_pte(struct msm_vm_bind_op *first, struct msm_vm_bind_op *next)
  * them as a single mapping.  Otherwise the prealloc_count() will not realize
  * they can share pagetable pages and vastly overcount.
  */
-static void
+static int
 vm_bind_prealloc_count(struct msm_vm_bind_job *job)
 {
 	struct msm_vm_bind_op *first = NULL, *last = NULL;
+	struct msm_gem_vm *vm = to_msm_vm(job->vm);
+	int ret;
 
 	for (int i = 0; i < job->nr_ops; i++) {
 		struct msm_vm_bind_op *op = &job->ops[i];
@@ -1122,6 +1130,20 @@ vm_bind_prealloc_count(struct msm_vm_bind_job *job)
 
 	/* Flush the remaining range: */
 	prealloc_count(job, first, last);
+
+	/*
+	 * Now that we know the needed amount to pre-alloc, throttle on pending
+	 * VM_BIND jobs if we already have too much pre-alloc memory in flight
+	 */
+	ret = wait_event_interruptible(
+			vm->prealloc_throttle.wait,
+			atomic_read(&vm->prealloc_throttle.in_flight) <= 1024);
+	if (ret)
+		return ret;
+
+	atomic_add(job->prealloc.count, &vm->prealloc_throttle.in_flight);
+
+	return 0;
 }
 
 /*
@@ -1412,7 +1434,9 @@ msm_ioctl_vm_bind(struct drm_device *dev, void *data, struct drm_file *file)
 	if (ret)
 		goto out_unlock;
 
-	vm_bind_prealloc_count(job);
+	ret = vm_bind_prealloc_count(job);
+	if (ret)
+		goto out_unlock;
 
 	struct drm_exec exec;
 	unsigned flags = DRM_EXEC_IGNORE_DUPLICATES | DRM_EXEC_INTERRUPTIBLE_WAIT;