linux-kernel - Re: [PATCH v3 1/2] x86, msr: allow rdmsr_safe_on

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Date:   Sat, 24 Mar 2018 11:50:23 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ingo Molnar <mingo@...nel.org>
cc:     Eric Dumazet <edumazet@...gle.com>, x86 <x86@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Hugh Dickins <hughd@...gle.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH v3 1/2] x86, msr: allow rdmsr_safe_on_cpu() to schedule

On Sat, 24 Mar 2018, Ingo Molnar wrote:
> * Eric Dumazet <edumazet@...gle.com> wrote:
> 
> > I noticed high latencies caused by a daemon periodically reading
> > various MSR on all cpus. KASAN kernels would see ~10ms latencies
> > simply reading one MSR. Even without KASAN, sending IPI to CPU
> > in deep sleep state or blocking hard IRQ in a a long section,
> > then waiting for the answer can consume hundreds of usec.
> > 
> > Converts rdmsr_safe_on_cpu() to use a completion instead
> > of busy polling.
> > 
> > Overall daemon cpu usage was reduced by 35 %,
> > and latencies caused by msr_read() disappeared.
> 
> What "daemon" is this and why is it reading MSRs?

Why? Just because it can ....