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<div class="moz-cite-prefix">On 12/22/2017 3:46 PM, Lionel
Landwerlin wrote:<br>
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<blockquote type="cite"
cite="mid:30446e83-3d0f-ae39-5c0d-a23bc4c89557@intel.com">
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<div class="moz-cite-prefix">On 22/12/17 09:30, Sagar Arun Kamble
wrote:<br>
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<blockquote type="cite"
cite="mid:523e5349-2db7-747a-bea0-774227913592@intel.com">
<p><br>
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<div class="moz-cite-prefix">On 12/21/2017 6:29 PM, Lionel
Landwerlin wrote:<br>
</div>
<blockquote type="cite"
cite="mid:62813062-eba1-0fa2-1959-6abf19e3dcae@intel.com">
<div class="moz-cite-prefix">Some more findings I made while
playing with this series & GPUTop.<br>
Turns out the 2ms drift per second is due to timecounter.
Adding the delta this way :<br>
<br>
<a class="moz-txt-link-freetext"
href="https://github.com/djdeath/linux/commit/7b002cb360483e331053aec0f98433a5bd5c5c3f#diff-9b74bd0cfaa90b601d80713c7bd56be4R607"
moz-do-not-send="true">https://github.com/djdeath/linux/commit/7b002cb360483e331053aec0f98433a5bd5c5c3f#diff-9b74bd0cfaa90b601d80713c7bd56be4R607</a><br>
<br>
Eliminates the drift.</div>
</blockquote>
I see two imp. changes 1. approximation of start time during
init_timecounter 2. overflow handling in delta accumulation.<br>
With these incorporated, I guess timecounter should also work in
same fashion.<br>
</blockquote>
<br>
I think the arithmetic in timecounter is inherently lossy and
that's why we're seeing a drift.</blockquote>
Could you share details about platform, scenario in which 2ms drift
per second is being seen with timecounter.<br>
I did not observe this on SKL.<br>
<blockquote type="cite"
cite="mid:30446e83-3d0f-ae39-5c0d-a23bc4c89557@intel.com"> Could
we be using it wrong?<br>
<br>
</blockquote>
if we use two changes highlighted above with timecounter maybe we
will get same results as your current implementation.<br>
<blockquote type="cite"
cite="mid:30446e83-3d0f-ae39-5c0d-a23bc4c89557@intel.com"> In the
patch above, I think there is still a drift because of the
potential fractional part loss at every delta we add.<br>
But it should only be a fraction of a nanosecond multiplied by the
number of reports over a period of time.<br>
With a report every 1us, that should still be much less than a 1ms
of drift over 1s.<br>
<br>
</blockquote>
timecounter interface takes care of fractional parts so that should
help us.<br>
we can either go with timecounter or our own implementation provided
conversions are precise.<br>
<blockquote type="cite"
cite="mid:30446e83-3d0f-ae39-5c0d-a23bc4c89557@intel.com"> We can
probably do better by always computing the clock using the entire
delta rather than the accumulated delta.<br>
<br>
</blockquote>
issue is that the reported clock cycles in the OA report is 32bits
LSB of GPU TS whereas counter is 36bits. Hence we will need to<br>
accumulate the delta. ofc there is assumption that two reports can't
be spaced with count value of 0xffffffff apart.<br>
<blockquote type="cite"
cite="mid:30446e83-3d0f-ae39-5c0d-a23bc4c89557@intel.com">
<blockquote type="cite"
cite="mid:523e5349-2db7-747a-bea0-774227913592@intel.com">
<blockquote type="cite"
cite="mid:62813062-eba1-0fa2-1959-6abf19e3dcae@intel.com">
<div class="moz-cite-prefix"> Timelines of perf i915
tracepoints & OA reports now make a lot more sense.<br>
<br>
There is still the issue that reading the CPU clock &
the RCS timestamp is inherently not atomic. So there is a
delta there.<br>
I think we should add a new i915 perf record type to express
the delta that we measure this way :<br>
<br>
<a class="moz-txt-link-freetext"
href="https://github.com/djdeath/linux/commit/7b002cb360483e331053aec0f98433a5bd5c5c3f#diff-9b74bd0cfaa90b601d80713c7bd56be4R2475"
moz-do-not-send="true">https://github.com/djdeath/linux/commit/7b002cb360483e331053aec0f98433a5bd5c5c3f#diff-9b74bd0cfaa90b601d80713c7bd56be4R2475</a><br>
<br>
So that userspace knows there might be a global offset
between the 2 times and is able to present it.<br>
</div>
</blockquote>
agree on this. Delta ns1-ns0 can be interpreted as max drift.<br>
<blockquote type="cite"
cite="mid:62813062-eba1-0fa2-1959-6abf19e3dcae@intel.com">
<div class="moz-cite-prefix"> Measurement on my KBL system
were in the order of a few microseconds (~30us).<br>
I guess we might be able to setup the correlation point
better (masking interruption?) to reduce the delta.<br>
</div>
</blockquote>
already using spin_lock. Do you mean NMI?<br>
</blockquote>
<br>
I don't actually know much on this point.<br>
if spin_lock is the best we can do, then that's it :)<br>
<br>
<blockquote type="cite"
cite="mid:523e5349-2db7-747a-bea0-774227913592@intel.com">
<blockquote type="cite"
cite="mid:62813062-eba1-0fa2-1959-6abf19e3dcae@intel.com">
<div class="moz-cite-prefix"> <br>
Thanks,<br>
<br>
-<br>
Lionel<br>
<br>
<br>
On 07/12/17 00:57, Robert Bragg wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAMou1-2Z7=A_GBcD9a5AvjRGM3_bG-ezoZJnGYvXkrCqqrmT1w@mail.gmail.com">
<div dir="ltr"><br>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Thu, Dec 7, 2017 at 12:48
AM, Robert Bragg <span dir="ltr"><<a
href="mailto:robert@sixbynine.org" target="_blank"
moz-do-not-send="true">robert@sixbynine.org</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div dir="ltr"><br>
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</blockquote>
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.8ex;border-left:1px #ccc solid;padding-left:1ex">
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<div class="gmail_extra">
<div class="gmail_quote">
<div> 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.<br>
</div>
</div>
</div>
</div>
</blockquote>
<div><br>
</div>
<div>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. <br>
</div>
<div><br>
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<div>- Robert<br>
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