RE: [RFC/RFT][PATCH v2] cpuidle: New timer events oriented governor for tickless systems

["Doug Smythies" <dsmythies@telus.net>]

On 2018.11.05 11:14 Giovanni Gherdovich wrote:
> On Sun, 2018-11-04 at 11:06 +0100, Rafael J. Wysocki wrote:
>>
>> You can use the cpu_idle trace point to correlate the selected state index
>> with the observed idle duration (that's what Doug did IIUC).
>
> True, that works; although I ended up slapping a tracepoint right at the
> beginning of the teo_update() and capturing the variables
> cpu_data->last_state, dev->last_residency and dev->cpu.
>
> I should have some plots to share soon. I really wanted to do in-kernel
> histograms with systemtap as opposed to collecting data with ftrace and doing
> post-processing, because I noticed that the latter approach generates lots of
> events and wakeups from idle on the cpu that handles the ftrace data. It's
> kind of a workload in itself and spoils the results.

I agree that we need to be careful not to influence the system we
are trying to acquire diagnostic data on via the act of acquiring
that data.

I did not find much, if any, effect of acquiring trace data during the
dbench with 12 clients test. Regardless I do the exact same test the exact
same way for the baseline reference kernel and the test kernel. To be
clear, I mean no effect while actually acquiring the trace samples.
Obviously there is a significant effect while the samples are eventually
written out to disk, after being acquired. But at that point, I don’t care.

For tests where I am also acquiring long term idle statistics, over many
hours, I never run a trace at the same time, and only sample the system once
per minute. For those test scenarios, when a trace is required, i.e. for
greater detail, it is done as an independent step. But yes, for my very high
idle state 0 entry/exit per unit time type tests, enabling trace has a very
significant effect on the system under test. I haven't figured out a way around that.
For example the test where ~6 gigabytes of trace data was collected in
2 minutes, at a cost of ~25% performance drop
(https://marc.info/?l=linux-pm&m=153897853630373&w=2)
For comparison, the 12 client Phoronix dbench test trace on kernel 4.20-rc1
(baseline reference for TEO V3 tests) was only 199 Megabytes in 10 minutes.

... Doug