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

[Tvrtko Ursulin]

On 08/07/2019 13:16, Chris Wilson wrote:
> 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.

 From gen8_configure_all_contexts, twice, or really recursively? If 
former isn't that solvable by simply skipping kernel contexts in the 
first loop?

>> 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.

There isn't some time cost associated with l3_remap calls when switching 
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.

My tree still has it? Is it out of date? Solution with keeping it on the 
list and skipping sounds more future proof if doable.

Regards,

Tvrtko