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

[Ville Syrjälä]

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@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx> 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@xxxxxxxxx> 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?

> 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