tracing: Up the hist stacktrace size from 16 to 31

Recording stacktraces is very useful, but the size of 16 deep is very
restrictive. For example, in seeing where tasks schedule out in a non
running state, the following can be used:

 ~# cd /sys/kernel/tracing
 ~# echo 'hist:keys=common_stacktrace:vals=hitcount if prev_state & 3' > events/sched/sched_switch/trigger
 ~# cat events/sched/sched_switch/hist
[..]
{ common_stacktrace:
         __schedule+0xdc0/0x1860
         schedule+0x27/0xd0
         schedule_timeout+0xb5/0x100
         wait_for_completion+0x8a/0x140
         xfs_buf_iowait+0x20/0xd0 [xfs]
         xfs_buf_read_map+0x103/0x250 [xfs]
         xfs_trans_read_buf_map+0x161/0x310 [xfs]
         xfs_btree_read_buf_block+0xa0/0x120 [xfs]
         xfs_btree_lookup_get_block+0xa3/0x1e0 [xfs]
         xfs_btree_lookup+0xea/0x530 [xfs]
         xfs_alloc_fixup_trees+0x72/0x570 [xfs]
         xfs_alloc_ag_vextent_size+0x67f/0x800 [xfs]
         xfs_alloc_vextent_iterate_ags.constprop.0+0x52/0x230 [xfs]
         xfs_alloc_vextent_start_ag+0x9d/0x1b0 [xfs]
         xfs_bmap_btalloc+0x2af/0x680 [xfs]
         xfs_bmapi_allocate+0xdb/0x2c0 [xfs]
} hitcount:          1
[..]

The above stops at 16 functions where knowing more would be useful. As the
allocated storage for stacks is the same for strings, and that size is 256
bytes, there is a lot of space not being used for stacktraces.

 16 * 8 = 128

Up the size to 31 (it requires the last slot to be zero, so it can't be 32).

Also change the BUILD_BUG_ON() to allow the size of the stacktrace storage
to be equal to the max size. One slot is used to hold the number of
elements in the stack.

  BUILD_BUG_ON((HIST_STACKTRACE_DEPTH + 1) * sizeof(long) >= STR_VAR_LEN_MAX);

Change that from ">=" to just ">", as now they are equal.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20260123105415.2be26bf4@gandalf.local.home
Reviewed-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: rostedt

kernel/trace/trace.h

@@ -128,7 +128,7 @@ enum trace_type {
 
 #define FAULT_STRING "(fault)"
 
-#define HIST_STACKTRACE_DEPTH	16
+#define HIST_STACKTRACE_DEPTH	31
 #define HIST_STACKTRACE_SIZE	(HIST_STACKTRACE_DEPTH * sizeof(unsigned long))
 #define HIST_STACKTRACE_SKIP	5
 

kernel/trace/trace_events_hist.c

@@ -3163,7 +3163,7 @@ static inline void __update_field_vars(struct tracing_map_elt *elt,
 	u64 var_val;
 
 	/* Make sure stacktrace can fit in the string variable length */
-	BUILD_BUG_ON((HIST_STACKTRACE_DEPTH + 1) * sizeof(long) >= STR_VAR_LEN_MAX);
+	BUILD_BUG_ON((HIST_STACKTRACE_DEPTH + 1) * sizeof(long) > STR_VAR_LEN_MAX);
 
 	for (i = 0, j = field_var_str_start; i < n_field_vars; i++) {
 		struct field_var *field_var = field_vars[i];