Re: [Intel-gfx] [PATCH 7/8] drm/i915: Disable tracing points on PREEMPT_RT

Jani Nikula
2021-12-14T15:41:32Z

On Tue, 14 Dec 2021, Steven Rostedt <rostedt@goodmis.org> wrote:
> On Tue, 14 Dec 2021 15:03:00 +0100
> Sebastian Andrzej Siewior <bigeasy@linutronix.de> 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_i915_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.
>
> Another way around this that I can see is if the data for the tracepoints
> can fit on the stack and add wrappers around the tracepoints. For example,
> looking at the first tracepoint in i915_trace.h:

FYI display portions of the file have been split to
display/intel_display_trace.[ch] in current drm-intel-next, headed for
v5.17 merge window.

BR,
Jani.

>
> TRACE_EVENT(intel_pipe_enable,
> TP_PROTO(struct intel_crtc *crtc),
> TP_ARGS(crtc),
>
> TP_STRUCT__entry(
> __array(u32, frame, 3)
> __array(u32, scanline, 3)
> __field(enum pipe, pipe)
> ),
> TP_fast_assign(
> struct drm_i915_private *dev_priv = to_i915(crtc->base.dev);
> struct intel_crtc *it__;
> for_each_intel_crtc(&dev_priv->drm, it__) {
> __entry->frame[it__->pipe] = intel_crtc_get_vblank_counter(it__);
> __entry->scanline[it__->pipe] = intel_get_crtc_scanline(it__);
> }
> __entry->pipe = crtc->pipe;
> ),
>
> TP_printk("pipe %c enable, pipe A: frame=%u, scanline=%u, pipe B: frame=%u, scanline=%u, pipe C: frame=%u, scanline=%u",
> pipe_name(__entry->pipe),
> __entry->frame[PIPE_A], __entry->scanline[PIPE_A],
> __entry->frame[PIPE_B], __entry->scanline[PIPE_B],
> __entry->frame[PIPE_C], __entry->scanline[PIPE_C])
> );
>
> We could modify this to be:
>
> TRACE_EVENT(intel_pipe_enable,
> TP_PROTO(u32 *frame, u32 *scanline, enum pipe),
> TP_ARGS(frame, scanline, pipe),
>
> TP_STRUCT__entry(
> __array(u32, frame, 3)
> __array(u32, scanline, 3)
> __field(enum pipe, pipe)
> ),
> TP_fast_assign(
> int i;
> for (i = 0; i < 3; i++) {
> __entry->frame[i] = frame[i];
> __entry->scanline[i] = scanline[i];
> }
> __entry->pipe = pipe;
> ),
>
> TP_printk("pipe %c enable, pipe A: frame=%u, scanline=%u, pipe B: frame=%u, scanline=%u, pipe C: frame=%u, scanline=%u",
> pipe_name(__entry->pipe),
> __entry->frame[PIPE_A], __entry->scanline[PIPE_A],
> __entry->frame[PIPE_B], __entry->scanline[PIPE_B],
> __entry->frame[PIPE_C], __entry->scanline[PIPE_C])
> );
>
>
> static inline void do_trace_intel_pipe(struct intel_crtc *crtc)
> {
> u32 frame[3];
> u32 scanline[3];
> enum pipe pipe;
>
> if (!trace_intel_pipe_enable_enabled())
> return;
>
> struct drm_i915_private *dev_priv = to_i915(crtc->base.dev);
> struct intel_crtc *it__;
> for_each_intel_crtc(&dev_priv->drm, it__) {
> frame[it__->pipe] = intel_crtc_get_vblank_counter(it__);
> scanline[it__->pipe] = intel_get_crtc_scanline(it__);
> }
>
> trace_intel_pipe(frame, scanline, crtc->pipe);
> }
>
>
> The trace_intel_pipe_enable_enabled() is a static_branch that will act the
> same as the nop of a trace event, so this will still not add overhead when
> not enabled.
>
> All the processing will be done outside the trace event allowing it to be
> preempted, and then when the trace event is executed, it will run quickly
> without taking any locks.
>
> Then have the code call do_trace_intel_pipe() instead of trace_intel_pipe()
> and this should fix the issue with preempt rt.
>
> -- Steve

--
Jani Nikula, Intel Open Source Graphics Center