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

[Chris Wilson <chris@chris-wilson.co.uk>]

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@intel.com>

For ctx->priority/ctx->priority_offset
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

At the end of the day, everything that is modifiable by context is going
to want cgroup constraint, but like priority_offset each will require
some thought as to how to express the constraint.

Interesting conundrum, and still we want a consistent interface for all
the gpus on a system.
-Chris
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