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

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Date:   Sat, 24 Mar 2018 09:09:46 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     x86 <x86@...nel.org>, lkml <linux-kernel@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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


* 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?

Thanks,

	Ingo