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

[Lionel Landwerlin <lionel.g.landwerlin@xxxxxxxxx>]

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@xxxxxxxxxxxxx> 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

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