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Subject: Re: [tip: sched/core] x86, sched: Add support for frequency invariance
From:   Chris Wilson <chris@chris-wilson.co.uk>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Doug Smythies <dsmythies@telus.net>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        x86 <x86@kernel.org>
To:     Giovanni Gherdovich <ggherdovich@suse.cz>,
        linux-kernel@vger.kernel.org, linux-tip-commits@vger.kernel.org,
        tip-bot2 for Giovanni Gherdovich <tip-bot2@linutronix.de>
Message-ID: <158567963079.5852.13300525696474233020@build.alporthouse.com>
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Date:   Tue, 31 Mar 2020 19:33:50 +0100
Sender: linux-kernel-owner@vger.kernel.org
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Quoting Giovanni Gherdovich (2020-03-31 19:11:25)
> Hello Chris,
> 
> thank you for catching this problem and sorry for the mess.
> 
> Until your message I wasn't aware that CPU0 can be hotplugged, but now that I
> check the feature is been there since v3.8 :/
> 
> The code assumes cpu0 is always there and I need to fix that.
> 
> It seems your report comes from executing an automated test suite, can you
> give me a link to the test sources and a hint on how to run it? I'd like to
> reproduce locally so that I make sure I correctly address this problem.

https://gitlab.freedesktop.org/drm/igt-gpu-tools/

It's an i915 test (so expects i915 running and root access to your
machine, with the intent of breaking your machine), but the cpu
hotplugging could be extracted

https://gitlab.freedesktop.org/drm/igt-gpu-tools/-/blob/master/tests/perf_pmu.c#L1153

since it's basically doing:
	i = 0
	while :; do
		test -e /sys/devices/system/cpu/cpu$i/online || break

		echo 0 > /sys/devices/system/cpu/cpu$i/online
		sleep .1
		echo 1 > /sys/devices/system/cpu/cpu$i/online

		i = $[[ $i + 1 ]]
	done
	dmesg

Possibly running that under perf stat to keep perf_event_open, or
something else that hooks up the perf cpu hotplug callbacks.

Hope that helps,
-Chris