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

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Message-ID: <20241021141832.3668264-1-ryan.roberts@arm.com>
Date: Mon, 21 Oct 2024 15:18:31 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Ryan Roberts <ryan.roberts@....com>,
	linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org
Subject: [PATCH v2] tracing: Make percpu stack trace buffer invariant to PAGE_SIZE

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@....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