On Thu, Aug 03, 2017 at 12:35:48PM -0700, Michel Thierry wrote:
> Hi,
>
> First sorry about the delay...
>
> On 7/20/2017 10:57 AM, Daniel Vetter wrote:
> > There's no reason to entirely wedge the gpu, for the minimal deadlock
> > bugfix we only need to unbreak/decouple the atomic commit from the gpu
> > reset. The simplest way to fix that is by replacing the
> > unconditional fence wait a the top of commit_tail by a wait which
> > completes either when the fences are done (normal case, or when a
> > reset doesn't need to touch the display state). Or when the gpu reset
> > needs to force-unblock all pending modeset states.
> >
> > The lesser source of deadlocks is when we try to pin a new framebuffer
> > and run into a stall. There's a bunch of places this can happen, like
> > eviction, changing the caching mode, acquiring a fence on older
> > platforms. And we can't just break the depency loop and keep going,
> > the only way would be to break out and restart. But the problem with
> > that approach is that we must stall for the reset to complete before
> > we grab any locks, and with the atomic infrastructure that's a bit
> > tricky. The only place is the ioctl code, and we don't want to insert
> > code into e.g. the BUSY ioctl. Hence for that problem just create a
> > critical section, and if any code is in there, wedge the GPU. For the
> > steady-state this should never be a problem.
> >
> > Note that in both cases TDR itself keeps working, so from a userspace
> > pov this trickery isn't observable. Users themselvs might spot a short
> > glitch while the rendering is catching up again, but that's still
> > better than pre-TDR where we've thrown away all the rendering,
> > including innocent batches. Also, this fixes the regression TDR
> > introduced of making gpu resets deadlock-prone when we do need to
> > touch the display.
> >
> > One thing I noticed is that gpu_error.flags seems to use both our own
> > wait-queue in gpu_error.wait_queue, and the generic wait_on_bit
> > facilities. Not entirely sure why this inconsistency exists, I just
> > picked one style.
> >
> > A possible future avenue could be to insert the gpu reset in-between
> > ongoing modeset changes, which would avoid the momentary glitch. But
> > that's a lot more work to implement in the atomic commit machinery,
> > and given that we only need this for pre-g4x hw, of questionable
> > utility just for the sake of polishing gpu reset even more on those
> > old boxes. It might be useful for other features though.
> >
> > v2: Rebase onto 4.13 with a s/wait_queue_t/struct wait_queue_entry/.
> >
> > v3: Really emabarrassing fixup, I checked the wrong bit and broke the
> > unbreak/wakeup logic.
> >
> > v4: Also handle deadlocks in pin_to_display.
> >
> > Cc: Chris Wilson <ch...@chris-wilson.co.uk>
> > Cc: Mika Kuoppala <mika.kuopp...@intel.com>
> > Cc: Joonas Lahtinen <joonas.lahti...@linux.intel.com>
> > Signed-off-by: Daniel Vetter <daniel.vet...@intel.com>
> > ---
> > drivers/gpu/drm/i915/i915_drv.h | 3 +++
> > drivers/gpu/drm/i915/intel_display.c | 45
> > +++++++++++++++++++++++++++++++-----
> > 2 files changed, 42 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_drv.h
> > b/drivers/gpu/drm/i915/i915_drv.h
> > index 07e98b07c5bc..9b055deca36d 100644
> > --- a/drivers/gpu/drm/i915/i915_drv.h
> > +++ b/drivers/gpu/drm/i915/i915_drv.h
> > @@ -1505,6 +1505,8 @@ struct i915_gpu_error {
> > /* Protected by the above dev->gpu_error.lock. */
> > struct i915_gpu_state *first_error;
> >
> > + atomic_t pending_fb_pin;
> > +
> > unsigned long missed_irq_rings;
> >
> > /**
> > @@ -1564,6 +1566,7 @@ struct i915_gpu_error {
> > unsigned long flags;
> > #define I915_RESET_BACKOFF 0
> > #define I915_RESET_HANDOFF 1
> > +#define I915_RESET_MODESET 2
> > #define I915_WEDGED (BITS_PER_LONG - 1)
> > #define I915_RESET_ENGINE (I915_WEDGED - I915_NUM_ENGINES)
> >
>
> Since you still need to resend this, could you also update the
> I915_RESET_ENGINE overflow check in i915_handle_error? i.e.:
>
> ---BUILD_BUG_ON(I915_RESET_HANDOFF >= I915_RESET_ENGINE);
> +++BUILD_BUG_ON(I915_RESET_MODESET >= I915_RESET_ENGINE);
>
> Not that we would have this problem anytime soon...
Yeah missed that, will fix.
> > diff --git a/drivers/gpu/drm/i915/intel_display.c
> > b/drivers/gpu/drm/i915/intel_display.c
> > index f6bd6282d7f7..63ea8d6b2ebd 100644
> > --- a/drivers/gpu/drm/i915/intel_display.c
> > +++ b/drivers/gpu/drm/i915/intel_display.c
> > @@ -2162,6 +2162,8 @@ intel_pin_and_fence_fb_obj(struct drm_framebuffer
> > *fb, unsigned int rotation)
> > */
> > intel_runtime_pm_get(dev_priv);
> >
> > + atomic_inc(&dev_priv->gpu_error.pending_fb_pin);
> > +
>
> Do we also need this in intel_overlay_do_put_image? It is also being called
> when the struct_mutex is held.
Indeed I missed that, will fix too and resubmit the pile (once we agree on
a comment for the previous patch).
Thanks, Daniel
>
>
> > vma = i915_gem_object_pin_to_display_plane(obj, alignment, &view);
> > if (IS_ERR(vma))
> > goto err;
> > @@ -2189,6 +2191,8 @@ intel_pin_and_fence_fb_obj(struct drm_framebuffer
> > *fb, unsigned int rotation)
> >
> > i915_vma_get(vma);
> > err:
> > + atomic_dec(&dev_priv->gpu_error.pending_fb_pin);
> > +
> > intel_runtime_pm_put(dev_priv);
> > return vma;
> > }
> > @@ -3471,11 +3475,14 @@ void intel_prepare_reset(struct drm_i915_private
> > *dev_priv)
> > !gpu_reset_clobbers_display(dev_priv))
> > return;
> >
> > - /* We have a modeset vs reset deadlock, defensively unbreak it.
> > - *
> > - * FIXME: We can do a _lot_ better, this is just a first
> > iteration.*/
> > - i915_gem_set_wedged(dev_priv);
> > - DRM_DEBUG_DRIVER("Wedging GPU to avoid deadlocks with pending
> > modeset updates\n");
> > + /* We have a modeset vs reset deadlock, defensively unbreak it. */
> > + set_bit(I915_RESET_MODESET, &dev_priv->gpu_error.flags);
> > + wake_up_all(&dev_priv->gpu_error.wait_queue);
> > +
> > + if (atomic_read(&dev_priv->gpu_error.pending_fb_pin)) {
> > + DRM_DEBUG_KMS("Modeset potentially stuck, unbreaking
> > through wedging\n");
> > + i915_gem_set_wedged(dev_priv);
> > + }
> >
> > /*
> > * Need mode_config.mutex so that we don't
> > @@ -3570,6 +3577,8 @@ void intel_finish_reset(struct drm_i915_private
> > *dev_priv)
> > drm_modeset_drop_locks(ctx);
> > drm_modeset_acquire_fini(ctx);
> > mutex_unlock(&dev->mode_config.mutex);
> > +
> > + clear_bit(I915_RESET_MODESET, &dev_priv->gpu_error.flags);
> > }
> >
> > static bool abort_flip_on_reset(struct intel_crtc *crtc)
> ^^^ this is the only reason of why we need a rebase.
>
> > @@ -12381,6 +12390,30 @@ static void
> > intel_atomic_helper_free_state_worker(struct work_struct *work)
> > intel_atomic_helper_free_state(dev_priv);
> > }
> >
> > +static void intel_atomic_commit_fence_wait(struct intel_atomic_state
> > *intel_state)
> > +{
> > + struct wait_queue_entry wait_fence, wait_reset;
> > + struct drm_i915_private *dev_priv = to_i915(intel_state->base.dev);
> > +
> > + init_wait_entry(&wait_fence, 0);
> > + init_wait_entry(&wait_reset, 0);
> > + for (;;) {
> > + prepare_to_wait(&intel_state->commit_ready.wait,
> > + &wait_fence, TASK_UNINTERRUPTIBLE);
> > + prepare_to_wait(&dev_priv->gpu_error.wait_queue,
> > + &wait_reset, TASK_UNINTERRUPTIBLE);
> > +
> > +
> > + if (i915_sw_fence_done(&intel_state->commit_ready)
> > + || test_bit(I915_RESET_MODESET,
> > &dev_priv->gpu_error.flags))
> > + break;
> > +
> > + schedule();
> > + }
> > + finish_wait(&intel_state->commit_ready.wait, &wait_fence);
> > + finish_wait(&dev_priv->gpu_error.wait_queue, &wait_reset);
> > +}
> > +
> > static void intel_atomic_commit_tail(struct drm_atomic_state *state)
> > {
> > struct drm_device *dev = state->dev;
> > @@ -12394,7 +12427,7 @@ static void intel_atomic_commit_tail(struct
> > drm_atomic_state *state)
> > unsigned crtc_vblank_mask = 0;
> > int i;
> >
> > - i915_sw_fence_wait(&intel_state->commit_ready);
> > + intel_atomic_commit_fence_wait(intel_state);
> >
> > drm_atomic_helper_wait_for_dependencies(state);
> >
>
> Looks good to me.
>
> -Michel
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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