On Thu, Jun 6, 2019 at 7:28 PM Daniel Vetter <daniel.vetter@xxxxxxxx> wrote: > > The issue we have is that the crc worker might fall behind. We've > tried to handle this by tracking both the earliest frame for which it > still needs to compute a crc, and the last one. Plus when the > crtc_state changes, we have a new work item, which are all run in > order due to the ordered workqueue we allocate for each vkms crtc. > > Trouble is there's been a few small issues in the current code: > - we need to capture frame_end in the vblank hrtimer, not in the > worker. The worker might run much later, and then we generate a lot > of crc for which there's already a different worker queued up. > - frame number might be 0, so create a new crc_pending boolean to > track this without confusion. > - we need to atomically grab frame_start/end and clear it, so do that > all in one go. This is not going to create a new race, because if we > race with the hrtimer then our work will be re-run. > - only race that can happen is the following: > 1. worker starts > 2. hrtimer runs and updates frame_end > 3. worker grabs frame_start/end, already reading the new frame_end, > and clears crc_pending > 4. hrtimer calls queue_work() > 5. worker completes > 6. worker gets re-run, crc_pending is false > Explain this case a bit better by rewording the comment. > > v2: Demote warning level output to debug when we fail to requeue, this > is expected under high load when the crc worker can't quite keep up. > > Cc: Shayenne Moura <shayenneluzmoura@xxxxxxxxx> > Cc: Rodrigo Siqueira <rodrigosiqueiramelo@xxxxxxxxx> > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx> > Cc: Haneen Mohammed <hamohammed.sa@xxxxxxxxx> > Cc: Daniel Vetter <daniel@xxxxxxxx> > --- > drivers/gpu/drm/vkms/vkms_crc.c | 27 +++++++++++++-------------- > drivers/gpu/drm/vkms/vkms_crtc.c | 9 +++++++-- > drivers/gpu/drm/vkms/vkms_drv.h | 2 ++ > 3 files changed, 22 insertions(+), 16 deletions(-) > > diff --git a/drivers/gpu/drm/vkms/vkms_crc.c b/drivers/gpu/drm/vkms/vkms_crc.c > index d7b409a3c0f8..66603da634fe 100644 > --- a/drivers/gpu/drm/vkms/vkms_crc.c > +++ b/drivers/gpu/drm/vkms/vkms_crc.c > @@ -166,16 +166,24 @@ void vkms_crc_work_handle(struct work_struct *work) > struct drm_plane *plane; > u32 crc32 = 0; > u64 frame_start, frame_end; > + bool crc_pending; > unsigned long flags; > > spin_lock_irqsave(&out->state_lock, flags); > frame_start = crtc_state->frame_start; > frame_end = crtc_state->frame_end; > + crc_pending = crtc_state->crc_pending; > + crtc_state->frame_start = 0; > + crtc_state->frame_end = 0; > + crtc_state->crc_pending = false; > spin_unlock_irqrestore(&out->state_lock, flags); > > - /* _vblank_handle() hasn't updated frame_start yet */ > - if (!frame_start || frame_start == frame_end) > - goto out; > + /* > + * We raced with the vblank hrtimer and previous work already computed > + * the crc, nothing to do. > + */ > + if (!crc_pending) > + return; I think this condition is not reachable because crc_pending will be filled with true in `vkms_vblank_simulate()` which in turn schedule the function `vkms_crc_work_handle()`. Just for checking, I tried to reach this condition by running kms_flip, kms_pipe_crc_basic, and kms_cursor_crc with two different VM setups[1], but I couldn't reach it. What do you think? [1] Qemu parameters VM1: -m 1G -smp cores=2,cpus=2 VM2: -enable-kvm -m 2G -smp cores=4,cpus=4 > drm_for_each_plane(plane, &vdev->drm) { > struct vkms_plane_state *vplane_state; > @@ -196,20 +204,11 @@ void vkms_crc_work_handle(struct work_struct *work) > if (primary_crc) > crc32 = _vkms_get_crc(primary_crc, cursor_crc); > > - frame_end = drm_crtc_accurate_vblank_count(crtc); > - > - /* queue_work can fail to schedule crc_work; add crc for > - * missing frames > + /* > + * The worker can fall behind the vblank hrtimer, make sure we catch up. > */ > while (frame_start <= frame_end) > drm_crtc_add_crc_entry(crtc, true, frame_start++, &crc32); I want to take this opportunity to ask about this while; It's not really specific to this patch. I have to admit that I never fully got the idea behind this 'while'; it looks like that we just fill out the missed frames with a repeated value. FWIU, `drm_crtc_add_crc_entry()` will add an entry with the CRC information for a frame, but in this case, we are adding the same CRC for a different set of frames. I agree that near frame has a similar CRC value, but could we rely on this all the time? What could happen if we have a great difference from the frame_start and frame_end? > - > -out: > - /* to avoid using the same value for frame number again */ > - spin_lock_irqsave(&out->state_lock, flags); > - crtc_state->frame_end = frame_end; > - crtc_state->frame_start = 0; > - spin_unlock_irqrestore(&out->state_lock, flags); > } > > static int vkms_crc_parse_source(const char *src_name, bool *enabled) > diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c b/drivers/gpu/drm/vkms/vkms_crtc.c > index 1bbe099b7db8..c727d8486e97 100644 > --- a/drivers/gpu/drm/vkms/vkms_crtc.c > +++ b/drivers/gpu/drm/vkms/vkms_crtc.c > @@ -30,13 +30,18 @@ static enum hrtimer_restart vkms_vblank_simulate(struct hrtimer *timer) > * has read the data > */ > spin_lock(&output->state_lock); > - if (!state->frame_start) > + if (!state->crc_pending) > state->frame_start = frame; > + else > + DRM_DEBUG_DRIVER("crc worker falling behind, frame_start: %llu, frame_end: %llu\n", > + state->frame_start, frame); > + state->frame_end = frame; > + state->crc_pending = true; > spin_unlock(&output->state_lock); > > ret = queue_work(output->crc_workq, &state->crc_work); > if (!ret) > - DRM_WARN("failed to queue vkms_crc_work_handle"); > + DRM_DEBUG_DRIVER("vkms_crc_work_handle already queued\n"); > } > > spin_unlock(&output->lock); > diff --git a/drivers/gpu/drm/vkms/vkms_drv.h b/drivers/gpu/drm/vkms/vkms_drv.h > index 81f1cfbeb936..3c7e06b19efd 100644 > --- a/drivers/gpu/drm/vkms/vkms_drv.h > +++ b/drivers/gpu/drm/vkms/vkms_drv.h > @@ -56,6 +56,8 @@ struct vkms_plane_state { > struct vkms_crtc_state { > struct drm_crtc_state base; > struct work_struct crc_work; > + > + bool crc_pending; > u64 frame_start; > u64 frame_end; > }; > -- > 2.20.1 > -- Rodrigo Siqueira https://siqueira.tech _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel