Re: [RFC 0/4] GPU/CPU timestamps correlation for relating OA samples with system events

From: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
To: Robert Bragg <robert@sixbynine.org>,
	Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Intel Graphics Development <intel-gfx@lists.freedesktop.org>
Subject: Re: [RFC 0/4] GPU/CPU timestamps correlation for relating OA samples with system events
Date: Thu, 21 Dec 2017 12:59:25 +0000	[thread overview]
Message-ID: <62813062-eba1-0fa2-1959-6abf19e3dcae@intel.com> (raw)
In-Reply-To: <CAMou1-2Z7=A_GBcD9a5AvjRGM3_bG-ezoZJnGYvXkrCqqrmT1w@mail.gmail.com>

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Some more findings I made while playing with this series & GPUTop.
Turns out the 2ms drift per second is due to timecounter. Adding the 
delta this way :

https://github.com/djdeath/linux/commit/7b002cb360483e331053aec0f98433a5bd5c5c3f#diff-9b74bd0cfaa90b601d80713c7bd56be4R607

Eliminates the drift. Timelines of perf i915 tracepoints & OA reports 
now make a lot more sense.

There is still the issue that reading the CPU clock & the RCS timestamp 
is inherently not atomic. So there is a delta there.
I think we should add a new i915 perf record type to express the delta 
that we measure this way :

https://github.com/djdeath/linux/commit/7b002cb360483e331053aec0f98433a5bd5c5c3f#diff-9b74bd0cfaa90b601d80713c7bd56be4R2475

So that userspace knows there might be a global offset between the 2 
times and is able to present it.
Measurement on my KBL system were in the order of a few microseconds 
(~30us).
I guess we might be able to setup the correlation point better (masking 
interruption?) to reduce the delta.

Thanks,

-
Lionel

On 07/12/17 00:57, Robert Bragg wrote:
>
>
> On Thu, Dec 7, 2017 at 12:48 AM, Robert Bragg <robert@sixbynine.org 
> <mailto:robert@sixbynine.org>> wrote:
>
>
>     at least from what I wrote back then it looks like I was seeing a
>     drift of a few milliseconds per second on SKL. I vaguely recall it
>     being much worse given the frequency constants we had for Haswell.
>
>
> Sorry I didn't actually re-read my own message properly before 
> referencing it :) Apparently the 2ms per second drift was for Haswell, 
> so presumably not quite so bad for SKL.
>
> - Robert
>
>
>
> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/intel-gfx

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