linux-kernel - Re: tip.today - scheduler bam boom crash (cpu hotplug)

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Date:   Mon, 27 Feb 2017 16:27:32 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Wanpeng Li <kernellwp@...il.com>, Mike Galbraith <efault@....de>,
        LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: tip.today - scheduler bam boom crash (cpu hotplug)



On 27/02/2017 14:04, Peter Zijlstra wrote:
>>>> This results in sched clock always unstable for kvm guest since there
>>>> is no invariant tsc cpuid bit exposed for kvm guest currently. 
>>> What the heck is KVM_FEATURE_CLOCKSOURCE_STABLE_BIT /
>>> PVCLOCK_TSC_STABLE_BIT about then?
>> It checks that all the bugs in the host have been ironed out, and that
>> the host itself supports invtsc.
> But what does it mean if that is not so? That is, will kvm_clock_read()
> still be stable even if !stable?

If kvmclock is !stable, nobody should have set that the sched clock to
stable, to begin with.

However, if kvmclock is stable, we know that the sched clock is stable.

Paolo