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Message-ID: <23b30f0a-866a-4c45-aa71-f0df1c2935b3@linux.ibm.com>
Date: Wed, 31 Jul 2024 11:56:07 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: "Nysal Jan K.A." <nysal@...ux.ibm.com>,
Michael Ellerman <mpe@...erman.id.au>,
Thomas Gleixner <tglx@...utronix.de>
Cc: Tyrel Datwyler <tyreld@...ux.ibm.com>,
Michal Suchanek
<msuchanek@...e.de>,
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>,
Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
Ard Biesheuvel <ardb@...nel.org>,
Eric DeVolder <eric.devolder@...cle.com>,
Sourabh Jain <sourabhjain@...ux.ibm.com>,
Stephen Rothwell <sfr@...b.auug.org.au>,
Laurent Dufour <ldufour@...ux.ibm.com>, linux-kernel@...r.kernel.org,
linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 1/2] cpu/SMT: Enable SMT only if a core is online
On 7/31/24 8:31 AM, Nysal Jan K.A. wrote:
> From: "Nysal Jan K.A" <nysal@...ux.ibm.com>
>
> If a core is offline then enabling SMT should not online CPUs of
> this core. By enabling SMT, what is intended is either changing the SMT
> value from "off" to "on" or setting the SMT level (threads per core) from a
> lower to higher value.
>
> On PowerPC the ppc64_cpu utility can be used, among other things, to
> perform the following functions:
>
> ppc64_cpu --cores-on # Get the number of online cores
> ppc64_cpu --cores-on=X # Put exactly X cores online
> ppc64_cpu --offline-cores=X[,Y,...] # Put specified cores offline
> ppc64_cpu --smt={on|off|value} # Enable, disable or change SMT level
>
> If the user has decided to offline certain cores, enabling SMT should
> not online CPUs in those cores. This patch fixes the issue and changes
> the behaviour as described, by introducing an arch specific function
> topology_is_core_online(). It is currently implemented only for PowerPC.
>
> Fixes: 73c58e7e1412 ("powerpc: Add HOTPLUG_SMT support")
> Reported-by: Tyrel Datwyler <tyreld@...ux.ibm.com>
> Closes: https://groups.google.com/g/powerpc-utils-devel/c/wrwVzAAnRlI/m/5KJSoqP4BAAJ
> Signed-off-by: Nysal Jan K.A <nysal@...ux.ibm.com>
> ---
> Documentation/ABI/testing/sysfs-devices-system-cpu | 3 ++-
> kernel/cpu.c | 12 +++++++++++-
> 2 files changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
> index 325873385b71..de725ca3be82 100644
> --- a/Documentation/ABI/testing/sysfs-devices-system-cpu
> +++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
> @@ -562,7 +562,8 @@ Description: Control Symmetric Multi Threading (SMT)
> ================ =========================================
>
> If control status is "forceoff" or "notsupported" writes
> - are rejected.
> + are rejected. Note that enabling SMT on PowerPC skips
> + offline cores.
>
> What: /sys/devices/system/cpu/cpuX/power/energy_perf_bias
> Date: March 2019
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 1209ddaec026..b1fd2a3db91a 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -2689,6 +2689,16 @@ int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval)
> return ret;
> }
>
> +/**
> + * Check if the core a CPU belongs to is online
> + */
> +#if !defined(topology_is_core_online)
> +static inline bool topology_is_core_online(unsigned int cpu)
> +{
> + return true;
> +}
> +#endif
> +
> int cpuhp_smt_enable(void)
> {
> int cpu, ret = 0;
> @@ -2699,7 +2709,7 @@ int cpuhp_smt_enable(void)
> /* Skip online CPUs and CPUs on offline nodes */
> if (cpu_online(cpu) || !node_online(cpu_to_node(cpu)))
> continue;
> - if (!cpu_smt_thread_allowed(cpu))
> + if (!cpu_smt_thread_allowed(cpu) || !topology_is_core_online(cpu))
> continue;
> ret = _cpu_up(cpu, 0, CPUHP_ONLINE);
> if (ret)
Reviewed-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
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