[PATCH 5/9] drm/i915: Enable i915 perf stream for Haswell OA unit

[Martin Peres]

On 20/04/16 17:23, Robert Bragg wrote:
> Gen graphics hardware can be set up to periodically write snapshots of
> performance counters into a circular buffer via its Observation
> Architecture and this patch exposes that capability to userspace via the
> i915 perf interface.
>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Signed-off-by: Robert Bragg <robert at sixbynine.org>
> Signed-off-by: Zhenyu Wang <zhenyuw at linux.intel.com>
> ---
>   drivers/gpu/drm/i915/i915_drv.h         |  56 +-
>   drivers/gpu/drm/i915/i915_gem_context.c |  24 +-
>   drivers/gpu/drm/i915/i915_perf.c        | 940 +++++++++++++++++++++++++++++++-
>   drivers/gpu/drm/i915/i915_reg.h         | 338 ++++++++++++
>   include/uapi/drm/i915_drm.h             |  70 ++-
>   5 files changed, 1408 insertions(+), 20 deletions(-)
>
> +
> +
> +	/* It takes a fairly long time for a new MUX configuration to
> +	 * be be applied after these register writes. This delay
> +	 * duration was derived empirically based on the render_basic
> +	 * config but hopefully it covers the maximum configuration
> +	 * latency...
> +	 */
> +	mdelay(100);

With such a HW and SW design, how can we ever expose hope to get any
kind of performance when we are trying to monitor different metrics on each
draw call? This may be acceptable for system monitoring, but it is 
problematic
for the GL extensions :s

Since it seems like we are going for a perf API, it means that for every 
change
of metrics, we need to flush the commands, wait for the GPU to be done, then
program the new set of metrics via an IOCTL, wait 100 ms, and then we may
resume rendering ... until the next change. We are talking about a 
latency of
6-7 frames at 60 Hz here... this is non-negligeable...

I understand that we have a ton of counters and we may hide latency by not
allowing using more than half of the counters for every draw call or 
frame, but
even then, this 100ms delay is killing this approach altogether.

To be honest, if it indeed is an HW bug, then the approach that Samuel 
Pitoiset
and I used for Nouveau involving pushing an handle representing a
pre-computed configuration to the command buffer so as a software method
can be ask the kernel to reprogram the counters with as little idle time as
possible, would be useless as waiting for the GPU to be idle would 
usually not
take more than a few ms... which is nothing compared to waiting 100ms.

So, now, the elephant in the room, how can it take that long to apply the
change? Are the OA registers double buffered (NVIDIA's are, so as we can
reconfigure and start monitoring multiple counters at the same time)?

Maybe this 100ms is the polling period and the HW does not allow changing
the configuration in the middle of a polling session. In this case, this 
delay
should be dependent on the polling frequency. But even then, I would really
hope that the HW would allow us to tear down everything, reconfigure and
start polling again without waiting for the next tick. If not possible, 
maybe we
can change the frequency for the polling clock to make the polling event 
happen
sooner.

HW delays are usually a few microseconds, not milliseconds, that really 
suggests
that something funny is happening and the HW design is not understood 
properly.
If the documentation has nothing on this and the HW teams cannot help, 
then I
suggest a little REing session. I really want to see this work land, but 
the way I see
it right now is that we cannot rely on it because of this bug. Maybe 
fixing this bug
would require changing the architecture, so better address it before 
landing the
patches.

Worst case scenario, do not hesitate to contact me if non of the proposed
explanation pans out, I will take the time to read through the OA 
material and try my
REing skills on it. As I said, I really want to see this upstream!

Sorry...

Martin