[PATCH v2] tracing: Make percpu stack trace buffer invariant to PAGE_SIZE

[PATCH v2] tracing: Make percpu stack trace buffer invariant to PAGE_SIZE

[Ryan Roberts]

Previously the size of "struct ftrace_stacks" depended upon PAGE_SIZE.
For the common 4K page size, on a 64-bit system, sizeof(struct
ftrace_stacks) was 32K. But for a 64K page size, sizeof(struct
ftrace_stacks) was 512K.

But ftrace stack usage requirements should be invariant to page size. So
let's redefine FTRACE_KSTACK_ENTRIES so that "struct ftrace_stacks" is
always sized at 32K for 64-bit and 16K for 32-bit.

As a side effect, it removes the PAGE_SIZE compile-time constant
assumption from this code, which is required to reach the goal of
boot-time page size selection.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
 kernel/trace/trace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 1c69ca1f1088..d4654943b580 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2914,7 +2914,7 @@ trace_function(struct trace_array *tr, unsigned long ip, unsigned long
 /* Allow 4 levels of nesting: normal, softirq, irq, NMI */
 #define FTRACE_KSTACK_NESTING	4

-#define FTRACE_KSTACK_ENTRIES	(PAGE_SIZE / FTRACE_KSTACK_NESTING)
+#define FTRACE_KSTACK_ENTRIES	(SZ_4K / FTRACE_KSTACK_NESTING)

 struct ftrace_stack {
 	unsigned long		calls[FTRACE_KSTACK_ENTRIES];
--
2.43.0

Re: [PATCH v2] tracing: Make percpu stack trace buffer invariant to PAGE_SIZE

[Ryan Roberts]

On 21/10/2024 15:18, Ryan Roberts wrote:
> Previously the size of "struct ftrace_stacks" depended upon PAGE_SIZE.
> For the common 4K page size, on a 64-bit system, sizeof(struct
> ftrace_stacks) was 32K. But for a 64K page size, sizeof(struct
> ftrace_stacks) was 512K.
> 
> But ftrace stack usage requirements should be invariant to page size. So
> let's redefine FTRACE_KSTACK_ENTRIES so that "struct ftrace_stacks" is
> always sized at 32K for 64-bit and 16K for 32-bit.
> 
> As a side effect, it removes the PAGE_SIZE compile-time constant
> assumption from this code, which is required to reach the goal of
> boot-time page size selection.

Just a polite bump on this... Does anyone have any comments?

Thanks,
Ryan

> 
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
>  kernel/trace/trace.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 1c69ca1f1088..d4654943b580 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -2914,7 +2914,7 @@ trace_function(struct trace_array *tr, unsigned long ip, unsigned long
>  /* Allow 4 levels of nesting: normal, softirq, irq, NMI */
>  #define FTRACE_KSTACK_NESTING	4
> 
> -#define FTRACE_KSTACK_ENTRIES	(PAGE_SIZE / FTRACE_KSTACK_NESTING)
> +#define FTRACE_KSTACK_ENTRIES	(SZ_4K / FTRACE_KSTACK_NESTING)
> 
>  struct ftrace_stack {
>  	unsigned long		calls[FTRACE_KSTACK_ENTRIES];
> --
> 2.43.0
> 

Re: [PATCH v2] tracing: Make percpu stack trace buffer invariant to PAGE_SIZE

[Steven Rostedt]

On Wed, 30 Oct 2024 08:50:16 +0000
Ryan Roberts <ryan.roberts@arm.com> wrote:

> On 21/10/2024 15:18, Ryan Roberts wrote:
> > Previously the size of "struct ftrace_stacks" depended upon PAGE_SIZE.
> > For the common 4K page size, on a 64-bit system, sizeof(struct
> > ftrace_stacks) was 32K. But for a 64K page size, sizeof(struct
> > ftrace_stacks) was 512K.
> > 
> > But ftrace stack usage requirements should be invariant to page size. So
> > let's redefine FTRACE_KSTACK_ENTRIES so that "struct ftrace_stacks" is
> > always sized at 32K for 64-bit and 16K for 32-bit.
> > 
> > As a side effect, it removes the PAGE_SIZE compile-time constant
> > assumption from this code, which is required to reach the goal of
> > boot-time page size selection.  
> 
> Just a polite bump on this... Does anyone have any comments?

No, it's in my queue. I've just been traveling.

-- Steve