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.

Cc: Masami Hiramatsu <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Link: https://lore.kernel.org/[email protected]
Signed-off-by: Ryan Roberts <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: Ryan Roberts

kernel/trace/trace.c

@@ -2898,7 +2898,7 @@ 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];