Re: [PATCH v7 1/4] cpu/SMT: Provide a default topology_is_primary_thread()

On 2024/10/30 22:55, Thomas Gleixner wrote:

On Wed, Oct 30 2024 at 20:54, Yicong Yang wrote:

quoted

 
+#ifndef topology_is_primary_thread
+#define topology_is_primary_thread topology_is_primary_thread

Please do not glue defines and functions together w/o a newline in between.

sure, will add a newline here.

quoted

+static inline bool topology_is_primary_thread(unsigned int cpu)
+{
+	/*
+	 * On SMT hotplug the primary thread of the SMT won't be disabled.
+	 * Architectures do have a special primary thread (e.g. x86) need
+	 * to override this function. Otherwise just make the first thread
+	 * in the SMT as the primary thread.
+	 */
+	return cpu == cpumask_first(topology_sibling_cpumask(cpu));

How is that supposed to work? Assume both siblings are offline, then the
sibling mask is empty and you can't boot the CPU anymore.

For architectures' using arch_topology, topology_sibling_cpumask() will at least
contain the tested CPU itself. This is initialized in
drivers/base/arch_topology.c:reset_cpu_topology(). So it won't be empty here.

Besides we don't need to check topology_is_primary_thread() at boot time:
-> cpu_up(cpu)
     cpu_bootable()
       if (cpu_smt_control == CPU_SMT_ENABLED &&
           cpu_smt_thread_allowed(cpu)) // will always return true if !CONFIG_SMT_NUM_THREADS_DYNAMIC
         return true; // we'll always return here and @cpu is always bootable

Also tested fine in practice.

Thanks.