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

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Message-ID: <CANn89iL+CHRi1HJ6Mp4u=ECnfAXV2qxXZj_BsQuv+uwheAfDDw@mail.gmail.com>
Date:   Tue, 19 Apr 2022 08:51:54 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>, linux-pm@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [patch 00/10] x86/cpu: Consolidate APERF/MPERF code

On Fri, Apr 15, 2022 at 12:19 PM Thomas Gleixner <tglx@...utronix.de> 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.
>

Thanks a lot for working on that Thomas.

I am not sure I will be able to backport this to a Google prodkernel,
as I guess there will be many merge conflicts.

Do you have by any chance this work available in a git branch ?

Thanks.



> 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(-)
>
>