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To:     Thomas Gleixner <tglx@linutronix.de>
From:   Chris Wilson <chris@chris-wilson.co.uk>
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Subject: Re: Linux 5.3-rc7
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Quoting Thomas Gleixner (2019-09-07 16:00:17)
> Does this only happen with that CPU0 hotplug stuff enabled or on CPUs other
> than CPU0 as well? That hotplug CPU0 stuff is a bandaid so I wouldn't be
> surprised if we broke that somehow.

If I ignore cpu0 in that test and so use

[  133.847187] smpboot: CPU 1 is now offline
[  134.861861] x86: Booting SMP configuration:
[  134.861875] smpboot: Booting Node 0 Processor 1 APIC 0x2
[  134.880218] smpboot: CPU 2 is now offline
[  135.893806] smpboot: Booting Node 0 Processor 2 APIC 0x1
[  135.935115] smpboot: CPU 3 is now offline
[  136.949760] smpboot: Booting Node 0 Processor 3 APIC 0x3

that has run for 10 minutes without failure, so it seems confined to
cpu0 hotplugging. All we are doing in the test to generate the hotplugs
is:

for (int cpu = 0;; cpu++) {
	char name[128];
	int cpufd;

	snprintf(name, sizeof(name),
		 "/sys/devices/system/cpu/cpu%d/online",
		 cpu), sizeof(name));
	cpufd = open(name, O_WRONLY);
	if (cpufd < 0)
		break;

	write(cpufd, "0", 2);
	usleep(1e6);
	write(cpufd, "1", 2);

	close(cpufd);
}

-Chris