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

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Date:   Fri, 15 Apr 2022 21:19:48 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, "Rafael J. Wysocki" <rafael@...nel.org>,
        linux-pm@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: [patch 00/10] x86/cpu: Consolidate APERF/MPERF code

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,

	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(-)