linux-kernel - Re: [PATCH v2] tracing: Make percpu stack trace buffer invariant to PAGE

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Message-ID: <42f825a2-ceb6-488a-8c63-fcc879cce096@arm.com>
Date: Wed, 30 Oct 2024 08:50:16 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Steven Rostedt <rostedt@...dmis.org>,
 Masami Hiramatsu <mhiramat@...nel.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v2] tracing: Make percpu stack trace buffer invariant to
 PAGE_SIZE

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@....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
>