[Intel-gfx] [PATCH 2/2] drm/i915/tracepoints: Remove DRM_I915_LOW_LEVEL_TRACEPOINTS Kconfig option

Tue Aug 21 12:06:54 UTC 2018

Quoting Kukanova, Svetlana (2018-08-13 16:44:49)
> Joonas, sorry for interfering; could you please explain more regarding the
> options for tracing scheduling events better than tracepoints?
> After scheduling moves to GuC tools will have to switch to something like
> GuC-logging; but while kmd does scheduling isn't kernel-tracing the best solution?
> I know gpuvis is not the only attempt to use tracepoints for the same purpose.
> (there're trace.pl and S.E.A. and of course VTune though it probably is not
> considered to be existing as it's not open source). 
> And assuming this movement towards GuC is it not too late to invent a
> completely new way to provide tools with scheduling info from kmd? 
> Could we just improve the existing way and let it live its last years\months? 

Hi,

You actually mentioned the prime reason why we should not go and
hastily make tracepoints a stable uAPI with regards to scheduling
information.

The scheduler's nature will be evolving when some of the scheduling
decisions are moved to GuC and the way how we get the information
will be changing at that point, so tracepoints will indeed be a
very bad mechanism for providing the information.

The kernel scheduler is definitely not going anywhere with the
introduction of more hardware scheduling capabilities, so it is a
misconception to think that the interface would need to be completely
different for when GuC is enabled.

> 
> gpuvis works w\o modifying kernel for AMDgpu showing HW queue and HW execution;
> it cosplays Microsoft GPUView which works out-of-the-box on Windows too.
> Thus it appears that intel gfx on linux is the most closed platform, not
> bothering of observability (or even bothering about how to forbid observability).

gpuvis is a developer tool. The tracepoints behind this configure
switch are way more low-level than what the gpuvis seems to support
for AMDGPU *at all*. They seem to stick to IOCTL level. So from what
I see, we should be on-par with the competition even without any special
kernel configuration. So lets not get things mixed up.

And I remind, the tool is not shipping anywhere really (except the AUR),
but just built from source by developers in need, and they seem to be just
fine with re-compiling the kernel (as there have been no requests).

Once there is an actual request to have some metrics from vanilla
kernels through some end-user tools (not a developer tool, like here),
I'll be glad to discuss about how to provide the information the best
for them in a stable manner.

> Not long ago the MediaSDK team diagnosed a problem with their workloads
> looking at VTune timelines - seeing the difference between the time request
> came to kmd and time it went runnable & comparing the queues on 2 engines they
> understood that their requests have dependencies that were definitely
> unexpected. MediaSDK reported the problem to driver people and it was fixed.
> 
> I can add Dmitry Rogozhkin to discussion if the usefulness of scheduling
> timeline in tools is questionable, as far as I remember this wasn't the only
> use case they had, I'm sure he can add more.

I'm well aware of the use cases. And Dmitry is well aware of the need
for an Open Source consumer for any requested stable uAPIs. And we don't
currently have that, so there's no disconnect on information.

There's just no Open Source tool to first design and then validate the
interfaces against. There's just the debugging tool which happens to
work currently, without any guarantees that next kernel version would
not cause a substantial rework of the interfacing code.

The interface discussion would probably start from a DRM subsystem
level, so that the tool would have an equivalent level of base
experience from all drivers.

Regards, Joonas

> 
> Thank you,
> Svetlana
> 
> -----Original Message-----
> From: Intel-gfx [mailto:intel-gfx-bounces at lists.freedesktop.org] On Behalf Of Joonas Lahtinen
> Sent: Monday, August 13, 2018 12:55 PM
> To: Chris Wilson <chris at chris-wilson.co.uk>; Intel-gfx at lists.freedesktop.org; Tvrtko Ursulin <tursulin at ursulin.net>; Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
> Subject: Re: [Intel-gfx] [PATCH 2/2] drm/i915/tracepoints: Remove DRM_I915_LOW_LEVEL_TRACEPOINTS Kconfig option
> 
> Quoting Tvrtko Ursulin (2018-08-08 15:56:01)
> > On 08/08/2018 13:42, Chris Wilson wrote:
> > > Quoting Tvrtko Ursulin (2018-08-08 13:13:08)
> > This is true, no disagreement. My point simply was that we can provide 
> > this info easily to anyone. There is a little bit of analogy with perf 
> > scheduler tracing/map etc.
> > 
> > > I don't see any value in giving the information away, just the cost. 
> > > If you can convince Joonas of its merit, and if we can define just 
> > > exactly what ABI it constitutes, then I'd be happy to be the one who 
> > > says "I told you so" in the future for a change.
> > 
> > I think Joonas was okay in principle that we soft-commit to _trying_ 
> > to keep _some_ tracepoint stable-ish (where it makes sense and after 
> > some discussion for each) if IGT also materializes which auto-pings us 
> > (via
> > CI) when we break one of them. But I may be misremembering so Joonas 
> > please comment.
> 
> Currently gpuvis, using these, seems to be only packaged in one AUR repo, and they do make a not in the wiki how you need to configure kernel for debugging. And there's been no apparent demand for them to have it in stock kernel.
> 
> And even when we do get demand for having gpuvis or another tool working from vanilla kernel, tracepoints being a rather tricky subject, I would start the discussion by going through alternative means of providing the information the tool needs and considering those.
> 
> So lets still keep this option as it was introduced. The whole "tracepoints as stable uAPI" idea is a can of worms which is only dug into when other options are exhausted.
> 
> Regards, Joonas
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