Re: [PATCH 05/16] drm/i915: Add unit tests for the breadcrumb rbtree, wakeups

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 07/12/2016 13:58, Chris Wilson wrote:
> Third retroactive test, make sure that the seqno waiters are woken.
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/intel_breadcrumbs.c | 171 +++++++++++++++++++++++++++++++
>  1 file changed, 171 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/intel_breadcrumbs.c b/drivers/gpu/drm/i915/intel_breadcrumbs.c
> index fc950f7ff322..1374a54e41c9 100644
> --- a/drivers/gpu/drm/i915/intel_breadcrumbs.c
> +++ b/drivers/gpu/drm/i915/intel_breadcrumbs.c
> @@ -966,11 +966,182 @@ static int igt_insert_complete(void *ignore)
>  	return err;
>  }
>
> +struct igt_wakeup {
> +	struct task_struct *tsk;
> +	atomic_t *ready, *set, *done;
> +	struct intel_engine_cs *engine;
> +	unsigned long flags;
> +	wait_queue_head_t *wq;
> +	u32 seqno;
> +};
> +
> +static int wait_atomic(atomic_t *p)
> +{
> +	schedule();
> +	return 0;
> +}
> +
> +static int wait_atomic_timeout(atomic_t *p)
> +{
> +	return schedule_timeout(10 * HZ) ? 0 : -ETIMEDOUT;
> +}
> +
> +static int igt_wakeup_thread(void *arg)
> +{
> +	struct igt_wakeup *w = arg;
> +	struct intel_wait wait;
> +
> +	while (!kthread_should_stop()) {
> +		DEFINE_WAIT(ready);
> +
> +		for (;;) {
> +			prepare_to_wait(w->wq, &ready, TASK_INTERRUPTIBLE);
> +			if (atomic_read(w->ready) == 0)
> +				break;
> +
> +			schedule();
> +		}
> +		finish_wait(w->wq, &ready);

Have to say this is the first time I've learnt about wake_up_atomic_t & 
co. You couldn't use wait_on_atomic_t instead of the loop above?

> +		if (atomic_dec_and_test(w->set))
> +			wake_up_atomic_t(w->set);
> +

Okay, all the threads have observed that all other threads have been 
started, yes?

> +		if (test_bit(0, &w->flags))
> +			break;

One thread failed to start = bailout.

Do you intend to use the flags for something more which precludes a more 
descriptive name for its single purpose here?

> +
> +		intel_wait_init(&wait, w->seqno);
> +		intel_engine_add_wait(w->engine, &wait);
> +		for (;;) {
> +			set_current_state(TASK_UNINTERRUPTIBLE);
> +			if (i915_seqno_passed(intel_engine_get_seqno(w->engine),
> +					      w->seqno))
> +				break;
> +
> +			schedule();
> +		}
> +		intel_engine_remove_wait(w->engine, &wait);
> +		__set_current_state(TASK_RUNNING);
> +
> +		if (atomic_dec_and_test(w->done))
> +			wake_up_atomic_t(w->done);
> +	}
> +
> +	if (atomic_dec_and_test(w->done))
> +		wake_up_atomic_t(w->done);

Must be a special reason done is decremented in the loop and outside the 
loop?

> +	return 0;
> +}
> +
> +static void igt_wake_all_sync(atomic_t *ready,
> +			      atomic_t *set,
> +			      atomic_t *done,
> +			      wait_queue_head_t *wq,
> +			      int count)
> +{
> +	atomic_set(set, count);
> +	atomic_set(done, count);
> +
> +	atomic_set(ready, 0);
> +	wake_up_all(wq);
> +
> +	wait_on_atomic_t(set, wait_atomic, TASK_UNINTERRUPTIBLE);
> +	atomic_set(ready, count);
> +}
> +
> +static int igt_wakeup(void *ignore)
> +{
> +	const int state = TASK_UNINTERRUPTIBLE;
> +	struct intel_engine_cs *engine;
> +	struct igt_wakeup *waiters;
> +	DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
> +	const int count = 4096;
> +	const u32 max_seqno = count / 4;
> +	atomic_t ready, set, done;
> +	int err = -ENOMEM;
> +	int n, step;
> +
> +	engine = mock_engine("mock");
> +	if (!engine)
> +		goto out;
> +
> +	waiters = drm_malloc_gfp(count, sizeof(*waiters), GFP_TEMPORARY);
> +	if (!waiters)
> +		goto out_engines;
> +
> +	atomic_set(&ready, count);
> +	for (n = 0; n < count; n++) {
> +		waiters[n].wq = &wq;
> +		waiters[n].ready = &ready;
> +		waiters[n].set = &set;
> +		waiters[n].done = &done;
> +		waiters[n].engine = engine;
> +		waiters[n].flags = 0;
> +
> +		waiters[n].tsk = kthread_run(igt_wakeup_thread, &waiters[n],
> +					     "i915/igt:%d", n);
> +		if (IS_ERR(waiters[n].tsk))
> +			goto out_waiters;
> +
> +		get_task_struct(waiters[n].tsk);
> +	}
> +

It is time to start documenting the tests I think via nice comments at 
strategic places. Probably a short commentary on the test as a whole and 
then separately at interesting steps.

> +	for (step = 1; step <= max_seqno; step <<= 1) {
> +		u32 seqno;
> +
> +		for (n = 0; n < count; n++)
> +			waiters[n].seqno = 1 + get_random_int() % max_seqno;

So you have 4096 waiters but some are waiting on the same seqno, since 
there are at most 1024 unique seqnos. Took me a while to figure this one 
out.

> +
> +		mock_seqno_advance(engine, 0);
> +		igt_wake_all_sync(&ready, &set, &done, &wq, count);
> +
> +		for (seqno = 1; seqno <= max_seqno + step; seqno += step) {

First step wakes up all seqnos one by one, the other steps do it in 
chunks with a larger and larger skip. All the way to waking the whole 
bunch at once?

> +			usleep_range(50, 500);

Why sleep? It should work without it, no?

> +			mock_seqno_advance(engine, seqno);
> +		}
> +		GEM_BUG_ON(intel_engine_get_seqno(engine) < 1 + max_seqno);
> +
> +		err = wait_on_atomic_t(&done, wait_atomic_timeout, state);
> +		if (err) {
> +			pr_err("Timed out waiting for %d remaining waiters\n",
> +			       atomic_read(&done));
> +			break;
> +		}
> +
> +		err = check_rbtree_empty(engine);
> +		if (err)
> +			break;
> +	}
> +
> +out_waiters:
> +	for (n = 0; n < count; n++) {
> +		if (IS_ERR(waiters[n].tsk))
> +			break;
> +
> +		set_bit(0, &waiters[n].flags);
> +	}
> +
> +	igt_wake_all_sync(&ready, &set, &done, &wq, n);
> +	wait_on_atomic_t(&done, wait_atomic, state);

C-O-M-M-E-N-T-S! :D

> +
> +	for (n = 0; n < count; n++) {
> +		if (IS_ERR(waiters[n].tsk))
> +			break;
> +
> +		kthread_stop(waiters[n].tsk);
> +		put_task_struct(waiters[n].tsk);
> +	}
> +
> +	drm_free_large(waiters);
> +out_engines:
> +	kfree(engine);
> +out:
> +	return err;
> +}
> +
>  int intel_breadcrumbs_selftest(void)
>  {
>  	static const struct i915_subtest tests[] = {
>  		SUBTEST(igt_random_insert_remove),
>  		SUBTEST(igt_insert_complete),
> +		SUBTEST(igt_wakeup),
>  	};
>
>  	return i915_subtests(tests, NULL);
>

Phew, looks mostly OK. I think only one thing I am unsure of.

This was quite smart, please start adding comments when you come up with 
similar things. :)

Regards,

Tvrtko
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