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Message-ID: <20240612185046.1826891-1-nysal@linux.ibm.com>
Date: Thu, 13 Jun 2024 00:20:36 +0530
From: "Nysal Jan K.A." <nysal@...ux.ibm.com>
To: Michael Ellerman <mpe@...erman.id.au>,
Thomas Gleixner <tglx@...utronix.de>
Cc: Tyrel Datwyler <tyreld@...ux.ibm.com>, Michal Suchanek <msuchanek@...e.de>,
"Nysal Jan K.A" <nysal@...ux.ibm.com>,
Nicholas Piggin <npiggin@...il.com>,
Christophe Leroy <christophe.leroy@...roup.eu>,
"Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
Peter Zijlstra <peterz@...radead.org>,
Laurent Dufour <ldufour@...ux.ibm.com>, linuxppc-dev@...ts.ozlabs.org,
linux-kernel@...r.kernel.org
Subject: [PATCH 0/2] Skip offline cores when enabling SMT on PowerPC
From: "Nysal Jan K.A" <nysal@...ux.ibm.com>
After the addition of HOTPLUG_SMT support for PowerPC [1] there was a
regression reported [2] when enabling SMT. On a system with at least
one offline core, when enabling SMT, the expectation is that no CPUs
of offline cores are made online.
On a POWER9 system with 4 cores in SMT4 mode:
$ ppc64_cpu --info
Core 0: 0* 1* 2* 3*
Core 1: 4* 5* 6* 7*
Core 2: 8* 9* 10* 11*
Core 3: 12* 13* 14* 15*
Turn only one core on:
$ ppc64_cpu --cores-on=1
$ ppc64_cpu --info
Core 0: 0* 1* 2* 3*
Core 1: 4 5 6 7
Core 2: 8 9 10 11
Core 3: 12 13 14 15
Change the SMT level to 2:
$ ppc64_cpu --smt=2
$ ppc64_cpu --info
Core 0: 0* 1* 2 3
Core 1: 4 5 6 7
Core 2: 8 9 10 11
Core 3: 12 13 14 15
As expected we see only two CPUs of core 0 are online
Change the SMT level to 4:
$ ppc64_cpu --smt=4
$ ppc64_cpu --info
Core 0: 0* 1* 2* 3*
Core 1: 4* 5* 6* 7*
Core 2: 8* 9* 10* 11*
Core 3: 12* 13* 14* 15*
The CPUs of offline cores are made online. If a core is offline then
enabling SMT should not online CPUs of this core. An arch specific
function topology_is_core_online() is proposed to address this.
Another approach is to check the topology_sibling_cpumask() for any
online siblings. This avoids the need for an arch specific function
but is less efficient and more importantly this introduces a change
in existing behaviour on other architectures.
What is the expected behaviour on x86 when enabling SMT and certain cores
are offline?
[1] https://lore.kernel.org/lkml/20230705145143.40545-1-ldufour@linux.ibm.com/
[2] https://groups.google.com/g/powerpc-utils-devel/c/wrwVzAAnRlI/m/5KJSoqP4BAAJ
Nysal Jan K.A (2):
cpu/SMT: Enable SMT only if a core is online
powerpc/topology: Check if a core is online
arch/powerpc/include/asm/topology.h | 13 +++++++++++++
kernel/cpu.c | 12 +++++++++++-
2 files changed, 24 insertions(+), 1 deletion(-)
base-commit: c760b3725e52403dc1b28644fb09c47a83cacea6
--
2.35.3
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