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

[Tvrtko Ursulin <tursulin@ursulin.net>]

On 17/06/2024 11:07, Sebastian Andrzej Siewior wrote:
> 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 did a re-test but am not 100% certain yet. CI looks frustratingly 
noisy at the moment.

igt@debugfs_test@read_all_entries appears to be a fluke which is not new.

But igt@gem_exec_parallel@engines@basic from the latest run seem new.

So I queued another re-test.

> 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.

Ack, thanks!

Regards,

Tvrtko

> + */
> +#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