[Intel-gfx] [PATCH 1/2] drm/i915/oa: Reconfigure contexts on the fly

[Chris Wilson]

Quoting Tvrtko Ursulin (2019-07-08 13:10:34)
> 
> On 08/07/2019 12:19, Chris Wilson wrote:
> > Avoid a global idle barrier by reconfiguring each context by rewriting
> > them with MI_STORE_DWORD from the kernel context.
> > 
> > v2: We only need to determine the desired register values once, they are
> > the same for all contexts.
> > 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
> > ---
> >   drivers/gpu/drm/i915/gem/i915_gem_context.c |   2 +
> >   drivers/gpu/drm/i915/gt/intel_lrc.c         |   7 +-
> >   drivers/gpu/drm/i915/i915_perf.c            | 248 +++++++++++++++-----
> >   3 files changed, 195 insertions(+), 62 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > index e367dce2a696..1f0d10bb88c1 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > @@ -624,7 +624,9 @@ i915_gem_context_create_kernel(struct drm_i915_private *i915, int prio)
> >       ctx->sched.priority = I915_USER_PRIORITY(prio);
> >       ctx->ring_size = PAGE_SIZE;
> >   
> > +     /* Isolate the kernel context from prying eyes and sticky fingers */
> >       GEM_BUG_ON(!i915_gem_context_is_kernel(ctx));
> > +     list_del_init(&ctx->link);
> 
> Why exactly?

Otherwise we recursively try to modify the context.

> Any repercussions for i915_sysfs/i915_l3_write? debugfs I gather you 
> won't care about?

No, because what matters for those is user contexts.
 
> Should adding to i915->contexts.list be done from gem_context_register? 
> What remains not a kernel context, but on a list?

And I also plan to pull it under its own mutex.
 
> Won't preempt context be missed in re-configuration?

What preempt-context? :-p And I would skip kernel_context if I could, but
for whatever reason oa events are being sent to userspace even while the
GPU is idle and igt expects the regular tick.
-Chris