Re: [PATCH v2 3/8] drm/i915: Don't check for atomic context on PREEMPT_RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2024-06-14 13:19:25 [+0100], Tvrtko Ursulin wrote:
> > So the question is why do you need to know if the context is atomic?
> > The only impact is avoiding disabling preemption. Is it that important
> > to avoid it?
> > If so would cant_migrate() work? It requires CONFIG_DEBUG_ATOMIC_SLEEP=y
> > to do the trick.
> 
> ... catching misuse of atomic wait helpers step 2 - are you calling it from
> a non-atomic context without the real need. So should use the non-atomic
> helper instead.
> 
> When i915 development was very active and with a lot of contributors it was
> beneficial to catch these things which code review would easily miss.
> 
> Now that the pace is much, much slower, it is probably not very important.
> So this patch is acceptable for what I am concerned and:
> 
> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
> 
> Actually please also add the PREEMPT_RT angle to the comment above
> _WAIT_FOR_ATOMIC_CHECK. Sometimes lines change and git blame makes it hard
> to find the commit text.

Do you want me the repost the series? Are the bots happy enough?
I have the following as far this patch:

------->8--------------

The !in_atomic() check in _wait_for_atomic() triggers on PREEMPT_RT
because the uncore::lock is a spinlock_t and does not disable
preemption or interrupts.

Changing the uncore:lock to a raw_spinlock_t doubles the worst case
latency on an otherwise idle testbox during testing.

Ignore _WAIT_FOR_ATOMIC_CHECK() on PREEMPT_RT.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Link: https://lore.kernel.org/all/20211006164628.s2mtsdd2jdbfyf7g@linutronix.de/
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 drivers/gpu/drm/i915/i915_utils.h | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_utils.h b/drivers/gpu/drm/i915/i915_utils.h
index 06ec6ceb61d57..f0d3c5cdc1b1b 100644
--- a/drivers/gpu/drm/i915/i915_utils.h
+++ b/drivers/gpu/drm/i915/i915_utils.h
@@ -273,8 +273,13 @@ wait_remaining_ms_from_jiffies(unsigned long timestamp_jiffies, int to_wait_ms)
 						   (Wmax))
 #define wait_for(COND, MS)		_wait_for((COND), (MS) * 1000, 10, 1000)
 
-/* If CONFIG_PREEMPT_COUNT is disabled, in_atomic() always reports false. */
-#if defined(CONFIG_DRM_I915_DEBUG) && defined(CONFIG_PREEMPT_COUNT)
+/*
+ * If CONFIG_PREEMPT_COUNT is disabled, in_atomic() always reports false.
+ * On PREEMPT_RT the context isn't becoming atomic because it is used in an
+ * interrupt handler or because a spinlock_t is acquired. This leads warnings
+ * which don't occur otherwise and is therefore disabled.
+ */
+#if defined(CONFIG_DRM_I915_DEBUG) && defined(CONFIG_PREEMPT_COUNT) && !defined(CONFIG_PREEMPT_RT)
 # define _WAIT_FOR_ATOMIC_CHECK(ATOMIC) WARN_ON_ONCE((ATOMIC) && !in_atomic())
 #else
 # define _WAIT_FOR_ATOMIC_CHECK(ATOMIC) do { } while (0)

> Regards,
> 
> Tvrtko

Sebastian