Re: [PATCH v2 0/3] drm: commit_work scheduling

[Rob Clark <robdclark@gmail.com>]

On Thu, Oct 8, 2020 at 1:24 AM Ville Syrjälä
<ville.syrjala@linux.intel.com> wrote:
>
> On Wed, Oct 07, 2020 at 09:44:09AM -0700, Rob Clark wrote:
> > On Mon, Oct 5, 2020 at 5:15 AM Ville Syrjälä
> > <ville.syrjala@linux.intel.com> wrote:
> > >
> > > On Fri, Oct 02, 2020 at 10:55:52AM -0700, Rob Clark wrote:
> > > > On Fri, Oct 2, 2020 at 4:05 AM Ville Syrjälä
> > > > <ville.syrjala@linux.intel.com> wrote:
> > > > >
> > > > > On Fri, Oct 02, 2020 at 01:52:56PM +0300, Ville Syrjälä wrote:
> > > > > > On Thu, Oct 01, 2020 at 05:25:55PM +0200, Daniel Vetter wrote:
> > > > > > > On Thu, Oct 1, 2020 at 5:15 PM Rob Clark <robdclark@gmail.com> wrote:
> > > > > > > >
> > > > > > > > I'm leaning towards converting the other drivers over to use the
> > > > > > > > per-crtc kwork, and then dropping the 'commit_work` from atomic state.
> > > > > > > > I can add a patch to that, but figured I could postpone that churn
> > > > > > > > until there is some by-in on this whole idea.
> > > > > > >
> > > > > > > i915 has its own commit code, it's not even using the current commit
> > > > > > > helpers (nor the commit_work). Not sure how much other fun there is.
> > > > > >
> > > > > > I don't think we want per-crtc threads for this in i915. Seems
> > > > > > to me easier to guarantee atomicity across multiple crtcs if
> > > > > > we just commit them from the same thread.
> > > > >
> > > > > Oh, and we may have to commit things in a very specific order
> > > > > to guarantee the hw doesn't fall over, so yeah definitely per-crtc
> > > > > thread is a no go.
> > > >
> > > > If I'm understanding the i915 code, this is only the case for modeset
> > > > commits?  I suppose we could achieve the same result by just deciding
> > > > to pick the kthread of the first CRTC for modeset commits.  I'm not
> > > > really so much concerned about parallelism for modeset.
> > >
> > > I'm not entirely happy about the random differences between modesets
> > > and other commits. Ideally we wouldn't need any.
> > >
> > > Anyways, even if we ignore modesets we still have the issue with
> > > atomicity guarantees across multiple crtcs. So I think we still
> > > don't want per-crtc threads, rather it should be thread for each
> > > commit.
> >
> > I don't really see any other way to solve the priority inversion other
> > than per-CRTC kthreads.
>
> What's the problem with just something like a dedicated commit
> thread pool?

partly, I was trying to avoid re-implementing workqueue.  And partly
the thread-pool approach seems like it would be harder for userspace
to find the tasks which need priority adjustment.

But each CRTC is essentially a FIFO, pageflip N+1 on a given CRTC will
happen after pageflip N.

BR,
-R

> > I've been thinking about it a bit more, and
> > my conclusion is:
> >
> > (1) There isn't really any use for the N+1'th commit to start running
> > before the kthread_work for the N'th commit completes, so I don't mind
> > losing the unbound aspect of the workqueue approach
> > (2) For cases where there does need to be serialization between
> > commits on different CRTCs, since there is a per-CRTC kthread, you
> > could achieve this with locking
> >
> > Since i915 isn't using the atomic helpers here, I suppose it is an
> > option for i915 to just continue doing what it is doing.
> >
> > And I could ofc just stop using the atomic commit helper and do the
> > kthreads thing in msm. But my first preference would be that the
> > commit helper does generally the right thing.
> >
> > BR,
> > -R
>
> --
> Ville Syrjälä
> Intel