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

[Pingfan Liu <piliu@redhat.com>]

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.


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)


Testing result on my Power9 machine with SMT=4

-1. taskset -c 4 bash -c 'echo c > /proc/sysrq-trigger'

kdump:/# cat /proc/meminfo | grep Percpu
Percpu:              896 kB
kdump:/# cat /sys/devices/system/cpu/possible
0


-2. taskset -c 5 bash -c 'echo c > /proc/sysrq-trigger'

kdump:/# cat /proc/meminfo | grep Percpu
Percpu:             1792 kB
kdump:/# cat /sys/devices/system/cpu/possible
0-1



-3. taskset -c 6 bash -c 'echo c > /proc/sysrq-trigger'

kdump:/# cat /proc/meminfo | grep Percpu
Percpu:             1792 kB
kdump:/# cat /sys/devices/system/cpu/possible
0,2


-4. taskset -c 7 bash -c 'echo c > /proc/sysrq-trigger'

kdump:/# cat /proc/meminfo | grep Percpu
Percpu:             1792 kB
kdump:/# cat /sys/devices/system/cpu/possible
0,3


Thanks,
Pingfan