[Intel-gfx] [PATCH v3 5/6] drm/i915: Introduce 'priority offset' for GPU contexts (v2)

[Chris Wilson]

Quoting Chris Wilson (2018-03-08 11:32:04)
> Quoting Matt Roper (2018-03-06 23:46:59)
> > There are cases where a system integrator may wish to raise/lower the
> > priority of GPU workloads being submitted by specific OS process(es),
> > independently of how the software self-classifies its own priority.
> > Exposing "priority offset" as an i915-specific cgroup parameter will
> > enable such system-level configuration.
> > 
> > Normally GPU contexts start with a priority value of 0
> > (I915_CONTEXT_DEFAULT_PRIORITY) and then may be adjusted up/down from
> > there via other mechanisms.  We'd like to provide a system-level input
> > to the priority decision that will be taken into consideration, even
> > when userspace later attempts to set an absolute priority value via
> > I915_CONTEXT_PARAM_PRIORITY.  The priority offset introduced here
> > provides a base value that will always be added to (or subtracted from)
> > the software's self-assigned priority value.
> > 
> > This patch makes priority offset a cgroup-specific value; contexts will
> > be created with a priority offset based on the cgroup membership of the
> > process creating the context at the time the context is created.  Note
> > that priority offset is assigned at context creation time; migrating a
> > process to a different cgroup or changing the offset associated with a
> > cgroup will only affect new context creation and will not alter the
> > behavior of existing contexts previously created by the process.
> > 
> > v2:
> >  - Rebase onto new cgroup_priv API
> >  - Use current instead of drm_file->pid to determine which process to
> >    lookup priority for. (Chris)
> >  - Don't forget to subtract priority offset in context_getparam ioctl to
> >    make it match setparam behavior. (Chris)
> > 
> > Signed-off-by: Matt Roper <matthew.d.roper at intel.com>
> 
> For ctx->priority/ctx->priority_offset
> Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>

As a reminder, we do have the question of how to bound ctx->priority +
ctx->priority_offset. Currently, outside of the user range is privileged
space reserved for the kernel (both above and below). Now we can move
those even further to accommodate multiple non-overlapping cgroups.
We also have the presumption that only privileged processes can raise a
priority, but I presume the cgroup property itself is protected.
-Chris