Re: [PATCH v4 4/9] drm/i915: Disable tracing points on PREEMPT_RT

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Jul 15, 2025 at 10:01:45AM +0200, Maarten Lankhorst wrote:
> Hey,
> 
> I believe the platforms that require spinlocks for reading registers all predate Xe,
> so it should be possible to enable tracepoints on Xe.
> 
> I do see there is still a path there that takes locks, through intel_crtc_get_vblank_counter -> drm_crtc_accurate_vblank_count().
> 
> I would recommend changing intel_crtc_get_vblank_counter to use drm_vblank_count(),
> which doesn't take locks.
> 
> This reduces vblank accuracy slightly on old platforms and gen11+ DSI, but I would say for most usecases it has the same accuracy,

The non-accurate version is completely useless here. So NAK.

> with the benefit of keeping tracepoints enabled. The only users for intel_crtc_get_vblank_counter() are debug functions.
> 
> With that change, something like: 
> "#if defined(CONFIG_PREEMPT_RT) && defined(I915) && !defined(NOTRACE)"
> 
> would disable tracepoints only on i915.
> 
> Kind regards,
> ~Maarten
> 
> Den 2025-07-14 kl. 17:39, skrev Sebastian Andrzej Siewior:
> > 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.
> > 
> > Acked-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
> > Reported-by: Luca Abeni <lucabe72@gmail.com>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > ---
> >  drivers/gpu/drm/i915/display/intel_display_trace.h | 4 ++++
> >  drivers/gpu/drm/i915/i915_trace.h                  | 4 ++++
> >  drivers/gpu/drm/i915/intel_uncore_trace.h          | 4 ++++
> >  3 files changed, 12 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_display_trace.h b/drivers/gpu/drm/i915/display/intel_display_trace.h
> > index 27ebc32cb61a5..a519d94700c36 100644
> > --- a/drivers/gpu/drm/i915/display/intel_display_trace.h
> > +++ b/drivers/gpu/drm/i915/display/intel_display_trace.h
> > @@ -13,6 +13,10 @@
> >  #if !defined(__INTEL_DISPLAY_TRACE_H__) || defined(TRACE_HEADER_MULTI_READ)
> >  #define __INTEL_DISPLAY_TRACE_H__
> >  
> > +#if defined(CONFIG_PREEMPT_RT) && !defined(NOTRACE)
> > +#define NOTRACE
> > +#endif
> > +
> >  #include <linux/string.h>
> >  #include <linux/string_helpers.h>
> >  #include <linux/types.h>
> > diff --git a/drivers/gpu/drm/i915/i915_trace.h b/drivers/gpu/drm/i915/i915_trace.h
> > index 7ed41ce9b7085..6b87ef6005c69 100644
> > --- a/drivers/gpu/drm/i915/i915_trace.h
> > +++ b/drivers/gpu/drm/i915/i915_trace.h
> > @@ -6,6 +6,10 @@
> >  #if !defined(_I915_TRACE_H_) || defined(TRACE_HEADER_MULTI_READ)
> >  #define _I915_TRACE_H_
> >  
> > +#if defined(CONFIG_PREEMPT_RT) && !defined(NOTRACE)
> > +#define NOTRACE
> > +#endif
> > +
> >  #include <linux/stringify.h>
> >  #include <linux/types.h>
> >  #include <linux/tracepoint.h>
> > diff --git a/drivers/gpu/drm/i915/intel_uncore_trace.h b/drivers/gpu/drm/i915/intel_uncore_trace.h
> > index f13ff71edf2db..3c67e267fb602 100644
> > --- a/drivers/gpu/drm/i915/intel_uncore_trace.h
> > +++ b/drivers/gpu/drm/i915/intel_uncore_trace.h
> > @@ -7,6 +7,10 @@
> >  #if !defined(__INTEL_UNCORE_TRACE_H__) || defined(TRACE_HEADER_MULTI_READ)
> >  #define __INTEL_UNCORE_TRACE_H__
> >  
> > +#if defined(CONFIG_PREEMPT_RT) && !defined(NOTRACE)
> > +#define NOTRACE
> > +#endif
> > +
> >  #include "i915_reg_defs.h"
> >  
> >  #include <linux/types.h>

-- 
Ville Syrjälä
Intel