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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Laurent Dufour <ldufour@linux.ibm.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        linux-kernel@vger.kernel.org
Cc:     linux-arch@vger.kernel.org, x86@kernel.org,
        dave.hansen@linux.intel.com, mingo@redhat.com, bp@alien8.de,
        linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH 8/9] powerpc: Add HOTPLUG_SMT support
In-Reply-To: <6e86aedb-9349-afd4-0bcb-1949e828f997@linux.ibm.com>
References: <20230524155630.794584-1-mpe@ellerman.id.au>
 <20230524155630.794584-8-mpe@ellerman.id.au>
 <5752a488-be54-61a0-6d18-647456abc4ee@linux.ibm.com>
 <6e86aedb-9349-afd4-0bcb-1949e828f997@linux.ibm.com>
Date:   Sat, 10 Jun 2023 23:10:02 +0200
Message-ID: <87h6rf81n9.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Thu, Jun 01 2023 at 18:19, Laurent Dufour wrote:
> @@ -435,12 +435,17 @@ void __init cpu_smt_disable(bool force)
>   * The decision whether SMT is supported can only be done after the full
>   * CPU identification. Called from architecture code.
>   */
> -void __init cpu_smt_check_topology(unsigned int num_threads)
> +void __init cpu_smt_check_topology(unsigned int num_threads,
> +				   unsigned int max_threads)
>  {
>  	if (!topology_smt_supported())
>  		cpu_smt_control = CPU_SMT_NOT_SUPPORTED;
>  
> -	cpu_smt_max_threads = num_threads;
> +	cpu_smt_max_threads = max_threads;
> +
> +	WARN_ON(num_threads > max_threads);
> +	if (num_threads > max_threads)
> +		num_threads = max_threads;

This does not work. The call site does:

> +	cpu_smt_check_topology(smt_enabled_at_boot, threads_per_core);

smt_enabled_at_boot is 0 when 'smt-enabled=off', which is not what the
hotplug core expects. If SMT is disabled it brings up the primary
thread, which means cpu_smt_num_threads = 1.

This needs more thoughts to avoid a completely inconsistent duct tape
mess.

Btw, the command line parser and the variable smt_enabled_at_boot being
type int allow negative number of threads too... Maybe not what you want.

Thanks,

        tglx