Re: [PATCH 1/2] drm/i915: Disable tracepoints for PREEMPT_RT

[Jani Nikula <jani.nikula@linux.intel.com>]

On Thu, 28 Aug 2025, Maarten Lankhorst <dev@lankhorst.se> wrote:
> Luca Abeni reported this:
> | BUG: scheduling while atomic: kworker/u8:2/15203/0x00000003
> | CPU: 1 PID: 15203 Comm: kworker/u8:2 Not tainted 4.19.1-rt3 #10
> | Call Trace:
> |  rt_spin_lock+0x3f/0x50
> |  gen6_read32+0x45/0x1d0 [i915]
> |  g4x_get_vblank_counter+0x36/0x40 [i915]
> |  trace_event_raw_event_i915_pipe_update_start+0x7d/0xf0 [i915]
>
> The tracing events use trace_intel_pipe_update_start() among other events
> use functions acquire spinlock_t locks which are transformed into
> sleeping locks on PREEMPT_RT. A few trace points use
> intel_get_crtc_scanline(), others use ->get_vblank_counter() wich also
> might acquire a sleeping locks on PREEMPT_RT.
> At the time the arguments are evaluated within trace point, preemption
> is disabled and so the locks must not be acquired on PREEMPT_RT.
>
> Based on this I don't see any other way than disable trace points on
> PREMPT_RT.
>
> [mlankhorst]
> The original patch was insufficient, and since the tracing
> infrastructure does not allow for partial disabling of tracepoints.
>
> Completely disable tracing for the entire i915 driver in PREEMPT_RT,
> a separate fix for display tracepoints on xe is added to make those
> work.
>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
> Reported-by: Luca Abeni <lucabe72@gmail.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Co-developed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>
> ---
>  drivers/gpu/drm/i915/Makefile | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
> index 8535434430728..ba825d7f6ddf5 100644
> --- a/drivers/gpu/drm/i915/Makefile
> +++ b/drivers/gpu/drm/i915/Makefile
> @@ -13,6 +13,11 @@ subdir-ccflags-$(CONFIG_DRM_I915_WERROR) += -Werror
>  # drivers. Define I915 when building i915.
>  subdir-ccflags-y += -DI915
>  
> +# Disable tracepoints on i915 for PREEMPT_RT, unfortunately
> +# it's an all or nothing flag. You cannot selectively disable
> +# only some tracepoints.

Should probably be a FIXME.

Acked-by: Jani Nikula <jani.nikula@intel.com>


> +subdir-ccflags-$(CONFIG_PREEMPT_RT) += -DNOTRACE
> +
>  subdir-ccflags-y += -I$(src)
>  
>  # Please keep these build lists sorted!

-- 
Jani Nikula, Intel