linux-kernel - Re: [RFC v4 1/1] selftests/cpuidle: Add support for cpuidle latency measurement

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Date:   Wed, 02 Sep 2020 18:25:24 +0300
From:   Artem Bityutskiy <dedekind1@...il.com>
To:     Pratik Rajesh Sampat <psampat@...ux.ibm.com>, rjw@...ysocki.net,
        daniel.lezcano@...aro.org, srivatsa@...il.mit.edu,
        shuah@...nel.org, npiggin@...il.com, ego@...ux.vnet.ibm.com,
        svaidy@...ux.ibm.com, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        pratik.r.sampat@...il.com
Subject: Re: [RFC v4 1/1] selftests/cpuidle: Add support for cpuidle latency
 measurement

On Wed, 2020-09-02 at 17:15 +0530, Pratik Rajesh Sampat wrote:
> Measure cpuidle latencies on wakeup to determine and compare with the
> advertsied wakeup latencies for each idle state.

It looks like the measurements include more than just C-state wake,
they also include the overhead of waking up the proces, context switch,
and potentially any interrupts that happen on that CPU. I am not saying
this is not interesting data, it surely is, but it is going to be
larger than you see in cpuidle latency tables. Potentially
significantly larger.

Therefore, I am not sure this program should be advertised as "cpuidle
measurement". It really measures the "IPI latency" in case of the IPI
method.

> A baseline measurement for each case of IPI and timers is taken at
> 100 percent CPU usage to quantify for the kernel-userpsace overhead
> during execution.

At least on Intel platforms, this will mean that the IPI method won't
cover deep C-states like, say, PC6, because one CPU is busy. Again, not
saying this is not interesting, just pointing out the limitation.

I was working on a somewhat similar stuff for x86 platforms, and I am
almost ready to publish that on github. I can notify you when I do so
if you are interested. But here is a small presentation of the approach
that I did on Plumbers last year:

https://youtu.be/Opk92aQyvt0?t=8266

(the link points to the start of my talk)