Re: [PATCH] Revert "ACPI: CPPC: Fix remaining for_each_possible_cpu() to use online CPUs"

[Sean Kelley <skelley@nvidia.com>]

On 4/16/26 1:52 AM, Jinjie Ruan wrote:
> This reverts commit 56eb0c0ed345da7815274aa821a8546a073d7e97, because
> this commit cause warning call trace below when concurrently bringing up
> and down two SMT threads of a physical core.
> 
> The issue timeline is as follows:
> 
> 1. when the system starts,
> 	cpufreq: cpu: 220, policy->related_cpus: 220-221, policy->cpus: 220-221
> 
> 2. Offline cpu 220 and cpu 221.
> 
> 3. Online cpu 220
> - cpu 221 is now offline, as acpi_get_psd_map() use for_each_online_cpu(),
>     so the cpu_data->shared_cpu_map, policy->cpus, and related_cpus has only
>     cpu 220.
> 	cpufreq: cpu: 220, related_cpus: 220, cpus: 220
> 
> 4. offline cpu 220
> 
> 5. online cpu 221, the below call trace occurs:
> - Because cpu 220 and cpu 221 share one policy, and policy->related_cpus
>    = 220 after step 3, so cpu 221 is not in policy->related_cpus
>    but per_cpu(cpufreq_cpu_data, cpu221) is not NULL.
> 
> The _PSD (P-State Dependency) defines the hardware-level dependency of
> frequency control across CPU cores. Since this relationship is a physical
> attribute of the hardware topology, it remains constant regardless of the
> online or offline status of the CPUs.
> 
> Using for_each_online_cpu() in acpi_get_psd_map() is problematic. If a
> CPU is offline, it will be excluded from the shared_cpu_map.
> Consequently, if that CPU is brought online later, the kernel will fail to
> recognize it as part of any shared frequency domain.
> 
> Switch back to for_each_possible_cpu() to ensure that all cores defined
> in the ACPI tables are correctly mapped into their respective performance
> domains from the start. This aligns with the logic of policy->related_cpus,
> which must encompass all potentially available cores in the domain to
> prevent logic gaps during CPU hotplug operations.


Yep, agree that using for_each_online_cpu() in acpi_get_psd_map()
drops valid domain members and breaks the hotplug case you described.

But a plain revert also re-exposes the nosmt bug. On systems where a
possible CPU is never probed, per_cpu(cpc_desc_ptr, i) is NULL and
acpi_get_psd_map() currently hits goto err_fault instead of just
skipping that CPU.

So I think the fix is to restore for_each_possible_cpu() for the PSD
map, but change the NULL case to continue:

--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -530,7 +530,7 @@

                 match_cpc_ptr = per_cpu(cpc_desc_ptr, i);
                 if (!match_cpc_ptr)
-                       goto err_fault;
+                       continue;

                 match_pdomain = &(match_cpc_ptr->domain_info);

That way offline CPUs with valid descriptors remain in shared_cpu_map
(fixing the hotplug trace), while never-probed CPUs are skipped
instead of failing map construction.

The send_pcc_cmd() hunk already does if (!desc) continue, so reverting
that loop back to for_each_possible_cpu() looks fine as-is.

Happy to send the continue fix as a patch on top, or please feel free
to fold it into yours if that makes sense.

Sean

> 
> How to reproduce, on arm64 machine with SMT support which use acpi cppc
> cpufreq driver:
> 
> 	bash test.sh 220 & bash test.sh 221 &
> 
> 	The test.sh is as below:
> 		while true
> 			do
> 			echo 0 > /sys/devices/system/cpu/cpu${1}/online
> 			sleep 0.5
> 			cat /sys/devices/system/cpu/cpu${1}/cpufreq/related_cpus
> 			echo 1 >  /sys/devices/system/cpu/cpu${1}/online
> 			cat /sys/devices/system/cpu/cpu${1}/cpufreq/related_cpus
> 		done
> 
> 	CPU: 221 PID: 1119 Comm: cpuhp/221 Kdump: loaded Not tainted 6.6.0debug+ #5
> 	Hardware name: To be filled by O.E.M. S920X20/BC83AMDA01-7270Z, BIOS 20.39 09/04/2024
> 	pstate: a1400009 (NzCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
> 	pc : cpufreq_online+0x8ac/0xa90
> 	lr : cpuhp_cpufreq_online+0x18/0x30
> 	sp : ffff80008739bce0
> 	x29: ffff80008739bce0 x28: 0000000000000000 x27: ffff28400ca32200
> 	x26: 0000000000000000 x25: 0000000000000003 x24: ffffd483503ff000
> 	x23: ffffd483504051a0 x22: ffffd48350024a00 x21: 00000000000000dd
> 	x20: 000000000000001d x19: ffff28400ca32000 x18: 0000000000000000
> 	x17: 0000000000000020 x16: ffffd4834e6a3fc8 x15: 0000000000000020
> 	x14: 0000000000000008 x13: 0000000000000001 x12: 00000000ffffffff
> 	x11: 0000000000000040 x10: ffffd48350430728 x9 : ffffd4834f087c78
> 	x8 : 0000000000000001 x7 : ffff2840092bdf00 x6 : ffffd483504264f0
> 	x5 : ffffd48350405000 x4 : ffff283f7f95cc60 x3 : 0000000000000000
> 	x2 : ffff53bc2f94b000 x1 : 00000000000000dd x0 : 0000000000000000
> 	Call trace:
> 	 cpufreq_online+0x8ac/0xa90
> 	 cpuhp_cpufreq_online+0x18/0x30
> 	 cpuhp_invoke_callback+0x128/0x580
> 	 cpuhp_thread_fun+0x110/0x1b0
> 	 smpboot_thread_fn+0x140/0x190
> 	 kthread+0xec/0x100
> 	 ret_from_fork+0x10/0x20
> 	---[ end trace 0000000000000000 ]---
> 
> Cc: stable@vger.kernel.org
> Fixes: 56eb0c0ed345 ("ACPI: CPPC: Fix remaining for_each_possible_cpu() to use online CPUs")
> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
> ---
>   drivers/acpi/cppc_acpi.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> index f0e513e9ed5d..9ae29f2c6db8 100644
> --- a/drivers/acpi/cppc_acpi.c
> +++ b/drivers/acpi/cppc_acpi.c
> @@ -362,7 +362,7 @@ static int send_pcc_cmd(int pcc_ss_id, u16 cmd)
>   end:
>   	if (cmd == CMD_WRITE) {
>   		if (unlikely(ret)) {
> -			for_each_online_cpu(i) {
> +			for_each_possible_cpu(i) {
>   				struct cpc_desc *desc = per_cpu(cpc_desc_ptr, i);
>   
>   				if (!desc)
> @@ -524,7 +524,7 @@ int acpi_get_psd_map(unsigned int cpu, struct cppc_cpudata *cpu_data)
>   	else if (pdomain->coord_type == DOMAIN_COORD_TYPE_SW_ANY)
>   		cpu_data->shared_type = CPUFREQ_SHARED_TYPE_ANY;
>   
> -	for_each_online_cpu(i) {
> +	for_each_possible_cpu(i) {
>   		if (i == cpu)
>   			continue;
>