linux-kernel - Re: [patch 00/10] x86/cpu: Consolidate APERF/MPERF code

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Message-ID: <20220419215656.GT4285@paulmck-ThinkPad-P17-Gen-1>
Date:   Tue, 19 Apr 2022 14:56:56 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>, linux-pm@...r.kernel.org,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [patch 00/10] x86/cpu: Consolidate APERF/MPERF code

On Fri, Apr 15, 2022 at 09:19:48PM +0200, Thomas Gleixner wrote:
> APERF/MPERF is utilized in two ways:
> 
>   1) Ad hoc readout of CPU frequency which requires IPIs
> 
>   2) Frequency scale calculation for frequency invariant scheduling which
>      reads APERF/MPERF on every tick.
> 
> These are completely independent code parts. Eric observed long latencies
> when reading /proc/cpuinfo which reads out CPU frequency via #1 and
> proposed to replace the per CPU single IPI with a broadcast IPI.
> 
> While this makes the latency smaller, it is not necessary at all because #2
> samples APERF/MPERF periodically, except on idle or isolated NOHZ full CPUs
> which are excluded from IPI already.
> 
> It could be argued that not all APERF/MPERF capable systems have the
> required BIOS information to enable frequency invariance support, but in
> practice most of them do. So the APERF/MPERF sampling can be made
> unconditional and just the frequency scale calculation for the scheduler
> excluded.
> 
> The following series consolidates that.

Acked-by: Paul E. McKenney <paulmck@...nel.org>

> Thanks,
> 
> 	tglx
> ---
>  arch/x86/include/asm/cpu.h       |    2 
>  arch/x86/include/asm/topology.h  |   17 -
>  arch/x86/kernel/acpi/cppc.c      |   28 --
>  arch/x86/kernel/cpu/aperfmperf.c |  474 +++++++++++++++++++++++++++++++--------
>  arch/x86/kernel/cpu/proc.c       |    2 
>  arch/x86/kernel/smpboot.c        |  358 -----------------------------
>  fs/proc/cpuinfo.c                |    6 
>  include/linux/cpufreq.h          |    1 
>  8 files changed, 405 insertions(+), 483 deletions(-)
> 
>