Date: Mon, 20 Jul 2015 17:02:40 +0200
From: Joerg Roedel <jroedel@suse.de>
To: Borislav Petkov <bp@alien8.de>
Cc: Joerg Roedel <joro@8bytes.org>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        x86@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86/smpboot: Check for cpu_active on cpu initialization
Message-ID: <20150720150240.GJ13082@suse.de>
References: <1437038237-16741-1-git-send-email-joro@8bytes.org>
 <20150720144619.GA9361@nazgul.tnic>
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On Mon, Jul 20, 2015 at 04:46:19PM +0200, Borislav Petkov wrote:
> On Thu, Jul 16, 2015 at 11:17:17AM +0200, Joerg Roedel wrote:
> > The reason is that the cpu_active bit for the new CPU
> > becomes visible significantly later than the cpu_online bit.
> 
> I see
> 
> void set_cpu_online(unsigned int cpu, bool online)
> {
> 	if (online) {
> 		cpumask_set_cpu(cpu, to_cpumask(cpu_online_bits));
> 		cpumask_set_cpu(cpu, to_cpumask(cpu_active_bits));
> 	} else {
> 
> which is called in start_secondary().
> 
> Do you mean that setting the bit in cpu_active_mask gets delayed soo
> much? Because it comes right after setting the bit in cpu_online_mask.

Yes, cpu_active becomes either set a lot later in a KVM guest, when the
host decides to preempt the vCPU right after setting cpu_online, but
before cpu_active is set, or on bare-metal.

I have seen a report where this happens on bare metal, when the change
to the cpu_active bit becomes visible on the other CPU significantly
later than the the cpu_online bit. This happened on a pretty big machine
with 88 cores.


	Joerg

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