[PATCH 01/10] drm/vkms: Fix crc worker races
Rodrigo Siqueira
rodrigosiqueiramelo at gmail.com
Wed Jun 12 13:33:11 UTC 2019
On Thu, Jun 6, 2019 at 7:28 PM Daniel Vetter <daniel.vetter at ffwll.ch> 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 at gmail.com>
> Cc: Rodrigo Siqueira <rodrigosiqueiramelo at gmail.com>
> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> Cc: Haneen Mohammed <hamohammed.sa at gmail.com>
> Cc: Daniel Vetter <daniel at ffwll.ch>
> ---
> 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
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