Re: [PATCH 1/3] drm/i915/pmu: Disable on PREEMPT_RT

[Tvrtko Ursulin <tursulin@ursulin.net>]

On 29/09/2026 20:01, Umesh Nerlige Ramappa wrote:
> From: Maarten Lankhorst <dev@lankhorst.se>
> 
> I tried to come up with alternative and lockless approaches
> to make pmu work, but since the perf events are run in
> atomic context on PREEMPT_RT, ordinary spinlocks no longer
> work and forcewake and register accesses become problematic.

If it is only those events put the checks into config_status and 
engine_event_status respectively. That will allow the non-problematic 
events to work.

> Disable perf events on PREEMPT_RT for now, without affecting
> !PREEMPT_RT users.
> 
> Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>
> ---
> v2: s/reeturn/return/
> ---
>   drivers/gpu/drm/i915/i915_pmu.c | 10 ++++++++++
>   1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_pmu.c b/drivers/gpu/drm/i915/i915_pmu.c
> index 1c3bafda9c70..9ea5626399e5 100644
> --- a/drivers/gpu/drm/i915/i915_pmu.c
> +++ b/drivers/gpu/drm/i915/i915_pmu.c
> @@ -1156,6 +1156,16 @@ void i915_pmu_register(struct drm_i915_private *i915)
>   	};
>   	int ret = -ENOMEM;
>   
> +	/*
> +	 * PREEMPT_RT has problems with forcewake. Forcewake is used by the
> +	 * engine 'busy' event when guc submission is enabled, and to measure
> +	 * rc6 residency.
> +	 *
> +	 * Disable pmu events on PREEMPT_RT entirely to prevent deadlocks.
> +	 */
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> +		return;

It would be nice to emit a suitable drm_notice here, equivalent to how 
the existing error path does if the registration unexpectedly fails.

Regards,

Tvrtko

> +
>   	spin_lock_init(&pmu->lock);
>   	hrtimer_setup(&pmu->timer, i915_sample, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
>   	init_rc6(pmu);