On 06/12, Daniel Vetter wrote:
> On Wed, Jun 12, 2019 at 10:33:11AM -0300, Rodrigo Siqueira wrote:
> > 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?
>
> thread A thread B
> 1. run vblank hrtimer
>
> 2. starts running crc work (from previous
> vblank)
>
> 3. spin_lock() -> gets stalled on the spin_lock() because
> thread A has it already
>
> 4. update frame_end (only in
> later patches, atm this is
> impossible). crc_pending is set
> already.
>
> 5. schedule_work: since the work
> is running already, this means it
> is scheduled to run once more.
>
> 6. spin_unlock
>
> 7. compute crc, clear crc_pending
> 8. work finishes
> 9. work gets run again
> 8. crc_pending=false
>
> Since the spin_lock critical section is _very_ short (less than 1 usec I
> bet), this race is very hard to hit.
First of all, thank you very much for all of your detailed explanation
and sorry for my delay to reply, I was 'processing' all of your
comments. I believe that I understood the issues related with this
patchset, I just want to check with you if the diagram and the cases
below make sense:
timer |------|------|------|------|------|...
Case 1: +----x +---x +-----x
Case 2: A +----------x
B +----x
At the top of this diagram, I illustrated the vblank period along the
time. In the bottom lines, I highlighted two cases; the '+' represents
when the worker is queued (queue_work()), and the 'x' denotes when the
CRC work finishes its data processing. Before describing each case from
the diagram, I want to highlight that I'm focused on these two snippets
of code:
static enum hrtimer_restart vkms_vblank_simulate(struct hrtimer *timer) {
[..]
spin_lock(&output->crc_lock);
[..] data [..]
spin_unlock(&output->crc_lock);
[..]
}
void vkms_crc_work_handle(struct work_struct *work){
[..]
spin_lock_irq(&out->crc_lock);
crtc_state->crc_pending = false;
[..] data [..]
spin_unlock_irq(&output->crc_lock);
[..]
}
Cases:
1) This is the best scenario; each CRC worker finishes before the next
vblank.
2) In this scenario, one of the CRC workers extends along multiple
vblanks. If worker A already collected the sensitive data inside
vkms_crc_work_handle(), worker A and B will finish without problems
(thanks to your changes). However, if for any reason, the worker A did
not start before the worker B, the new work will take care of its own
CRC and the CRC from worker A. Finally, since worker B will set
crc_pending equal false when the worker A starts, it'll just return
because of the following code:
if (!crc_pending)
return;
Make sense?
> Exercise: Figure out why schedule_work _must_ schedule the work item to
> re-run if it's running already. If it doesn't do that there's another
> race.
>
> >
> > [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?
>
> It's a cheap trick for slow cpu: If the crc work gets behind the vblank
> hrtimer, we need to somehow catch up. With real hw this is not possible,
> but with vkms we simulate the hw. The only quick way to catch up is to
> fill out the same crc for everything. It's a lie, it will make some
> kms_atomic tests fail, but it's the only thing we can really do. Aside
> from trying to make the crc computation code as fast as possible.
> -Daniel
>
> >
> > > -
> > > -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
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
--
Rodrigo Siqueira
https://siqueira.tech
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