Re: [RFC PATCH v1 18/57] trace: Remove PAGE_SIZE compile-time constant assumption

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 14 Oct 2024 11:58:25 +0100
Ryan Roberts <ryan.roberts@arm.com> wrote:

> To prepare for supporting boot-time page size selection, refactor code
> to remove assumptions about PAGE_SIZE being compile-time constant. Code
> intended to be equivalent when compile-time page size is active.
> 
> Convert BUILD_BUG_ON() BUG_ON() since the argument depends on PAGE_SIZE
> and its not trivial to test against a page size limit.
> 
> Redefine FTRACE_KSTACK_ENTRIES so that "struct ftrace_stacks" is always
> sized at 32K for 64-bit and 16K for 32-bit. It was previously defined in
> terms of PAGE_SIZE (and worked out at the quoted sizes for a 4K page
> size). But for 64K pages, the size expanded to 512K. Given the ftrace
> stacks should be invariant to page size, this seemed like a waste. As a
> side effect, it removes the PAGE_SIZE compile-time constant assumption
> from this code.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
> 
> ***NOTE***
> Any confused maintainers may want to read the cover note here for context:
> https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/
> 
>  kernel/trace/fgraph.c | 2 +-
>  kernel/trace/trace.c  | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
> index d7d4fb403f6f0..47aa5c8d8090e 100644
> --- a/kernel/trace/fgraph.c
> +++ b/kernel/trace/fgraph.c
> @@ -534,7 +534,7 @@ ftrace_push_return_trace(unsigned long ret, unsigned long func,
>  	if (!current->ret_stack)
>  		return -EBUSY;
>  
> -	BUILD_BUG_ON(SHADOW_STACK_SIZE % sizeof(long));
> +	BUG_ON(SHADOW_STACK_SIZE % sizeof(long));

Absolutely not!

BUG_ON() is in no way a substitution of any BUILD_BUG_ON(). BUILD_BUG_ON()
is a non intrusive way to see if something isn't lined up correctly, and
can fix it before you execute any code. BUG_ON() is the most intrusive way
to say something is wrong and you crash the system.

Not to mention, when function graph tracing is enabled, this gets triggered
for *every* function call! So I do not want any runtime test done. Every
nanosecond counts in this code path.

If anything, this needs to be moved to initialization and checked once, if
it fails, gives a WARN_ON() and disables function graph tracing.

-- Steve


>  
>  	/* Set val to "reserved" with the delta to the new fgraph frame */
>  	val = (FGRAPH_TYPE_RESERVED << FGRAPH_TYPE_SHIFT) | FGRAPH_FRAME_OFFSET;
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index c3b2c7dfadef1..0f2ec3d30579f 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -2887,7 +2887,7 @@ 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];