linux-kernel - [PATCH] tracing: Ensure optimized hashing works

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Message-ID: <20250923165908.2399256-1-mkoutny@suse.com>
Date: Tue, 23 Sep 2025 18:59:07 +0200
From: Michal Koutný <mkoutny@...e.com>
To: linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org
Cc: Michal Koutný <mkoutny@...e.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: [PATCH] tracing: Ensure optimized hashing works

If ever PID_MAX_DEFAULT changes, it must be compatible with tracing
hashmaps assumptions.

Link: https://lore.kernel.org/r/20240409110126.651e94cb@gandalf.local.home/
Signed-off-by: Michal Koutný <mkoutny@...e.com>
---
 kernel/trace/trace_sched_switch.c | 2 ++
 1 file changed, 2 insertions(+)

Drive-by flushing an old idea. Take it or leave it.

diff --git a/kernel/trace/trace_sched_switch.c b/kernel/trace/trace_sched_switch.c
index cb49f7279dc80..aabae7daaac91 100644
--- a/kernel/trace/trace_sched_switch.c
+++ b/kernel/trace/trace_sched_switch.c
@@ -243,6 +243,8 @@ int trace_create_savedcmd(void)
 int trace_save_cmdline(struct task_struct *tsk)
 {
 	unsigned tpid, idx;
+	/* so that map_pid_to_cmdline indexing is efficient */
+	BUILD_BUG_ON(!is_power_of_2(PID_MAX_DEFAULT));
 
 	/* treat recording of idle task as a success */
 	if (!tsk->pid)
-- 
2.51.0