Re: [PATCHv8 3/5] powerpc/setup: Handle the case when boot_cpuid greater than nr_cpus

[Hari Bathini <hbathini@linux.ibm.com>]

On 11/10/23 8:35 am, Pingfan Liu wrote:
> On Tue, Oct 10, 2023 at 01:56:13PM +0530, Hari Bathini wrote:
>>
>>
>> On 09/10/23 5:00 pm, Pingfan Liu wrote:
>>> If the boot_cpuid is smaller than nr_cpus, it requires extra effort to
>>> ensure the boot_cpu is in cpu_present_mask. This can be achieved by
>>> reserving the last quota for the boot cpu.
>>>
>>> Note: the restriction on nr_cpus will be lifted with more effort in the
>>> successive patches
>>>
>>> Signed-off-by: Pingfan Liu <piliu@redhat.com>
>>> Cc: Michael Ellerman <mpe@ellerman.id.au>
>>> Cc: Nicholas Piggin <npiggin@gmail.com>
>>> Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
>>> Cc: Mahesh Salgaonkar <mahesh@linux.ibm.com>
>>> Cc: Wen Xiong <wenxiong@linux.ibm.com>
>>> Cc: Baoquan He <bhe@redhat.com>
>>> Cc: Ming Lei <ming.lei@redhat.com>
>>> Cc: kexec@lists.infradead.org
>>> To: linuxppc-dev@lists.ozlabs.org
>>> ---
>>>    arch/powerpc/kernel/setup-common.c | 25 ++++++++++++++++++++++---
>>>    1 file changed, 22 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/powerpc/kernel/setup-common.c b/arch/powerpc/kernel/setup-common.c
>>> index 81291e13dec0..f9ef0a2666b0 100644
>>> --- a/arch/powerpc/kernel/setup-common.c
>>> +++ b/arch/powerpc/kernel/setup-common.c
>>> @@ -454,8 +454,8 @@ struct interrupt_server_node {
>>>    void __init smp_setup_cpu_maps(void)
>>>    {
>>>    	struct device_node *dn;
>>> -	int shift = 0, cpu = 0;
>>> -	int j, nthreads = 1;
>>> +	int terminate, shift = 0, cpu = 0;
>>> +	int j, bt_thread = 0, nthreads = 1;
>>>    	int len;
>>>    	struct interrupt_server_node *intserv_node, *n;
>>>    	struct list_head *bt_node, head;
>>> @@ -518,6 +518,7 @@ void __init smp_setup_cpu_maps(void)
>>>    			for (j = 0 ; j < nthreads; j++) {
>>>    				if (be32_to_cpu(intserv[j]) == boot_cpu_hwid) {
>>>    					bt_node = &intserv_node->node;
>>> +					bt_thread = j;
>>>    					found_boot_cpu = true;
>>>    					/*
>>>    					 * Record the round-shift between dt
>>> @@ -537,11 +538,21 @@ void __init smp_setup_cpu_maps(void)
>>>    	/* Select the primary thread, the boot cpu's slibing, as the logic 0 */
>>>    	list_add_tail(&head, bt_node);
>>>    	pr_info("the round shift between dt seq and the cpu logic number: %d\n", shift);
>>> +	terminate = nr_cpu_ids;
>>>    	list_for_each_entry(intserv_node, &head, node) {
>>> +		j = 0;
>>
>>> +		/* Choose a start point to cover the boot cpu */
>>> +		if (nr_cpu_ids - 1 < bt_thread) {
>>> +			/*
>>> +			 * The processor core puts assumption on the thread id,
>>> +			 * not to breach the assumption.
>>> +			 */
>>> +			terminate = nr_cpu_ids - 1;
>>
>> nthreads is anyway assumed to be same for all cores. So, enforcing
>> nr_cpu_ids to a minimum of nthreads (and multiple of nthreads) should
>> make the code much simpler without the need for above check and the
>> other complexities addressed in the subsequent patches...
>>
> 
> Indeed, this series can be splited into two partsk, [1-2/5] and [3-5/5].
> In [1-2/5], if smaller, the nr_cpu_ids is enforced to be equal to
> nthreads. I will make it align upward on nthreads in the next version.
> So [1-2/5] can be totally independent from the rest patches in this
> series.

Yup. Would prefer it that way.

>  From an engineer's perspective, [3-5/5] are added to maintain the
> nr_cpus semantics. (Finally, nr_cpus=1 can be achieved but requiring
> effort on other subsystem)

I understand it would be nice to maintain semantics but not worth the
complexity it brings, IMHO. So, my suggest would be to drop [3-5/5].

Thanks
Hari