We currently have $lx_per_cpu() which works fine for stuff that kernel
code would access via per_cpu(). But this doesn't work for stuff that
kernel code accesses via per_cpu_ptr():

(gdb) p $lx_per_cpu(node_data[1].node_zones[2]->per_cpu_pageset)
Cannot access memory at address 0xffff11105fbd6c28

This is because we take the address of the pointer and use that as the
offset, instead of using the stored value.

Add a GDB version that mirrors the kernel API, which uses the pointer
value.

To be consistent with per_cpu_ptr(), we need to return the pointer value
instead of dereferencing it for the user. Therefore, move the existing
dereference out of the per_cpu() Python helper and do that only in the
$lx_per_cpu() implementation.

Signed-off-by: Brendan Jackman <jackmanb@google.com>
---
 scripts/gdb/linux/cpus.py | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/scripts/gdb/linux/cpus.py b/scripts/gdb/linux/cpus.py
index 13eb8b3901b8fc5e2500138c7d204ac5f7c9858c..1f7e62efb38bf4ddee0058fea8634fd377317669 100644
--- a/scripts/gdb/linux/cpus.py
+++ b/scripts/gdb/linux/cpus.py
@@ -46,7 +46,7 @@ def per_cpu(var_ptr, cpu):
             # !CONFIG_SMP case
             offset = 0
     pointer = var_ptr.cast(utils.get_long_type()) + offset
-    return pointer.cast(var_ptr.type).dereference()
+    return pointer.cast(var_ptr.type)
 
 
 cpu_mask = {}
@@ -149,11 +149,28 @@ Note that VAR has to be quoted as string."""
         super(PerCpu, self).__init__("lx_per_cpu")
 
     def invoke(self, var, cpu=-1):
-        return per_cpu(var.address, cpu)
+        return per_cpu(var.address, cpu).dereference()
 
 
 PerCpu()
 
+class PerCpuPtr(gdb.Function):
+    """Return per-cpu pointer.
+
+$lx_per_cpu_ptr("VAR"[, CPU]): Return the per-cpu pointer called VAR for the
+given CPU number. If CPU is omitted, the CPU of the current context is used.
+Note that VAR has to be quoted as string."""
+
+    def __init__(self):
+        super(PerCpuPtr, self).__init__("lx_per_cpu_ptr")
+
+    def invoke(self, var, cpu=-1):
+        return per_cpu(var, cpu)
+
+
+PerCpuPtr()
+
+
 def get_current_task(cpu):
     task_ptr_type = task_type.get_type().pointer()
 

---
base-commit: f527c5c2abb60ddfd6b929d479b985071500419d
change-id: 20250219-lx-per-cpu-ptr-c9dfd3098721

Best regards,
-- 
Brendan Jackman <jackmanb@google.com>

On 19.02.25 16:40, Brendan Jackman wrote:
> We currently have $lx_per_cpu() which works fine for stuff that kernel
> code would access via per_cpu(). But this doesn't work for stuff that
> kernel code accesses via per_cpu_ptr():
> 
> (gdb) p $lx_per_cpu(node_data[1].node_zones[2]->per_cpu_pageset)
> Cannot access memory at address 0xffff11105fbd6c28
> 
> This is because we take the address of the pointer and use that as the
> offset, instead of using the stored value.
> 
> Add a GDB version that mirrors the kernel API, which uses the pointer
> value.
> 
> To be consistent with per_cpu_ptr(), we need to return the pointer value
> instead of dereferencing it for the user. Therefore, move the existing
> dereference out of the per_cpu() Python helper and do that only in the
> $lx_per_cpu() implementation.
> 
> Signed-off-by: Brendan Jackman <jackmanb@google.com>
> ---
>  scripts/gdb/linux/cpus.py | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/scripts/gdb/linux/cpus.py b/scripts/gdb/linux/cpus.py
> index 13eb8b3901b8fc5e2500138c7d204ac5f7c9858c..1f7e62efb38bf4ddee0058fea8634fd377317669 100644
> --- a/scripts/gdb/linux/cpus.py
> +++ b/scripts/gdb/linux/cpus.py
> @@ -46,7 +46,7 @@ def per_cpu(var_ptr, cpu):
>              # !CONFIG_SMP case
>              offset = 0
>      pointer = var_ptr.cast(utils.get_long_type()) + offset
> -    return pointer.cast(var_ptr.type).dereference()
> +    return pointer.cast(var_ptr.type)
>  
>  
>  cpu_mask = {}
> @@ -149,11 +149,28 @@ Note that VAR has to be quoted as string."""
>          super(PerCpu, self).__init__("lx_per_cpu")
>  
>      def invoke(self, var, cpu=-1):
> -        return per_cpu(var.address, cpu)
> +        return per_cpu(var.address, cpu).dereference()
>  
>  
>  PerCpu()
>  

Style nit: additional blank line here

> +class PerCpuPtr(gdb.Function):
> +    """Return per-cpu pointer.
> +
> +$lx_per_cpu_ptr("VAR"[, CPU]): Return the per-cpu pointer called VAR for the
> +given CPU number. If CPU is omitted, the CPU of the current context is used.
> +Note that VAR has to be quoted as string."""
> +
> +    def __init__(self):
> +        super(PerCpuPtr, self).__init__("lx_per_cpu_ptr")
> +
> +    def invoke(self, var, cpu=-1):
> +        return per_cpu(var, cpu)
> +
> +
> +PerCpuPtr()
> +
> +
>  def get_current_task(cpu):
>      task_ptr_type = task_type.get_type().pointer()
>  
> 
> ---
> base-commit: f527c5c2abb60ddfd6b929d479b985071500419d
> change-id: 20250219-lx-per-cpu-ptr-c9dfd3098721
> 
> Best regards,

Makes sense!

Reviewed-by: Jan Kiszka <jan.kiszka@siemens.com>

Jan

-- 
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Linux Expert Center