linux-kernel - [PATCH 0/2] Online new threads according to the current SMT level

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Message-Id: <20230331153905.31698-1-ldufour@linux.ibm.com>
Date:   Fri, 31 Mar 2023 17:39:03 +0200
From:   Laurent Dufour <ldufour@...ux.ibm.com>
To:     mpe@...erman.id.au, npiggin@...il.com, christophe.leroy@...roup.eu
Cc:     msuchanek@...e.de, nathanl@...ux.ibm.com,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH 0/2] Online new threads according to the current SMT level

When a new CPU is added, the kernel is activating all its threads. This
leads to weird, but functional, result when adding CPU on a SMT 4 system
for instance.

Here the newly added CPU 1 has 8 threads while the other one has 4 threads
active (system has been booted with the 'smt-enabled=4' kernel option):

ltcden3-lp12:~ # ppc64_cpu --info
Core   0:    0*    1*    2*    3*    4     5     6     7
Core   1:    8*    9*   10*   11*   12*   13*   14*   15*

This mixed SMT level is confusing end users and some application like
lparstat are reporting wrong values.

There is no SMT level recorded in the kernel, neither in user space. Such a
level could be helpful when adding new CPU or when optimizing the energy
efficiency. This series introduce a new SYS FS entry named 'pseries_smt' to
store the current SMT level.

The SMT level is provided in best effort, writing a new value into that
entry is only recording it into the kernel. This way, it can be used when
new CPU are onlined for instance. There is no real change to the CPU setup
when a value is written, no CPU are onlined or offlined.

At boot time `pseries_smt` is loaded with smt_enabled_at_boot which is
containing the SMT level set at boot time, even if no kernel option is
specified.

The change is specific to pseries since CPU hot-plug is only provided for
this platform.

The second patch of this series is implementing the change to online only
the right number of threads when a new CPU is added.

Laurent Dufour (2):
  pseries/smp: export the smt level in the SYS FS.
  powerpc/pseries/cpuhp: respect current SMT when adding new CPU

 arch/powerpc/platforms/pseries/hotplug-cpu.c | 18 ++++++---
 arch/powerpc/platforms/pseries/pseries.h     |  3 ++
 arch/powerpc/platforms/pseries/smp.c         | 39 ++++++++++++++++++++
 3 files changed, 55 insertions(+), 5 deletions(-)

-- 
2.40.0