Commit 8bdc5daa authored Mar 14, 2025 by Sebastian Andrzej Siewior Committed by Peter Zijlstra Mar 17, 2025

sched: Add a generic function to return the preemption string



The individual architectures often add the preemption model to the begin
of the backtrace. This is the case on X86 or ARM64 for the "die" case
but not for regular warning. With the addition of DYNAMIC_PREEMPT for
PREEMPT_RT we end up with CONFIG_PREEMPT and CONFIG_PREEMPT_RT set
simultaneously. That means that everyone who tried to add that piece of
information gets it wrong for PREEMPT_RT because PREEMPT is checked
first.

Provide a generic function which returns the current scheduling model
considering LAZY preempt and the current state of PREEMPT_DYNAMIC.

The resulting strings are:
┏━━━━━━━━━━━┳━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━┓
┃   Model   ┃  -RT -DYN    ┃     +RT -DYN      ┃     -RT +DYN       ┃     +RT +DYN      ┃
┡━━━━━━━━━━━╇━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━┩
│NONE       │ NONE         │ n/a               │ PREEMPT(none)      │ n/a               │
├───────────┼──────────────┼───────────────────┼────────────────────┼───────────────────┤
│VOLUNTARY  │ VOLUNTARY    │ n/a               │ PREEMPT(voluntary) │ n/a               │
├───────────┼──────────────┼───────────────────┼────────────────────┼───────────────────┤
│FULL       │ PREEMPT      │ PREEMPT_RT        │ PREEMPT(full)      │ PREEMPT_{RT,full} │
├───────────┼──────────────┼───────────────────┼────────────────────┼───────────────────┤
│LAZY       │ PREEMPT_LAZY │ PREEMPT_{RT,LAZY} │ PREEMPT(lazy)      │ PREEMPT_{RT,lazy} │
└───────────┴──────────────┴───────────────────┴────────────────────┴───────────────────┘

[ The dynamic building of the string can lead to an empty string if the
  function is invoked simultaneously on two CPUs. ]

Co-developed-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Signed-off-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Co-developed-by: "Steven Rostedt (Google)" <rostedt@goodmis.org>
Signed-off-by: "Steven Rostedt (Google)" <rostedt@goodmis.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Link: https://lore.kernel.org/r/20250314160810.2373416-2-bigeasy@linutronix.de

parent 4bc45824

include/linux/preempt.h

+2 −0

Original line number	Diff line number	Diff line
		@@ -515,6 +515,8 @@ static inline bool preempt_model_rt(void)
		return IS_ENABLED(CONFIG_PREEMPT_RT);
		}

		extern const char *preempt_model_str(void);

		/*
		* Does the preemption model allow non-cooperative preemption?
		*

kernel/sched/core.c

+47 −0

Original line number	Diff line number	Diff line
		@@ -7646,10 +7646,57 @@ PREEMPT_MODEL_ACCESSOR(lazy);

		#else /* !CONFIG_PREEMPT_DYNAMIC: */

		#define preempt_dynamic_mode -1

		static inline void preempt_dynamic_init(void) { }

		#endif /* CONFIG_PREEMPT_DYNAMIC */

		const char *preempt_modes[] = {
		"none", "voluntary", "full", "lazy", NULL,
		};

		const char *preempt_model_str(void)
		{
		bool brace = IS_ENABLED(CONFIG_PREEMPT_RT) &&
		(IS_ENABLED(CONFIG_PREEMPT_DYNAMIC) \|\|
		IS_ENABLED(CONFIG_PREEMPT_LAZY));
		static char buf[128];

		if (IS_ENABLED(CONFIG_PREEMPT_BUILD)) {
		struct seq_buf s;

		seq_buf_init(&s, buf, sizeof(buf));
		seq_buf_puts(&s, "PREEMPT");

		if (IS_ENABLED(CONFIG_PREEMPT_RT))
		seq_buf_printf(&s, "%sRT%s",
		brace ? "_{" : "_",
		brace ? "," : "");

		if (IS_ENABLED(CONFIG_PREEMPT_DYNAMIC)) {
		seq_buf_printf(&s, "(%s)%s",
		preempt_dynamic_mode > 0 ?
		preempt_modes[preempt_dynamic_mode] : "undef",
		brace ? "}" : "");
		return seq_buf_str(&s);
		}

		if (IS_ENABLED(CONFIG_PREEMPT_LAZY)) {
		seq_buf_printf(&s, "LAZY%s",
		brace ? "}" : "");
		return seq_buf_str(&s);
		}

		return seq_buf_str(&s);
		}

		if (IS_ENABLED(CONFIG_PREEMPT_VOLUNTARY_BUILD))
		return "VOLUNTARY";

		return "NONE";
		}

		int io_schedule_prepare(void)
		{
		int old_iowait = current->in_iowait;

kernel/sched/debug.c

+6 −4

Original line number	Diff line number	Diff line
		@@ -244,11 +244,13 @@ static ssize_t sched_dynamic_write(struct file filp, const char __user ubuf,

		static int sched_dynamic_show(struct seq_file m, void v)
		{
		static const char * preempt_modes[] = {
		"none", "voluntary", "full", "lazy",
		};
		int j = ARRAY_SIZE(preempt_modes) - !IS_ENABLED(CONFIG_ARCH_HAS_PREEMPT_LAZY);
		int i = IS_ENABLED(CONFIG_PREEMPT_RT) * 2;
		int j;

		/* Count entries in NULL terminated preempt_modes */
		for (j = 0; preempt_modes[j]; j++)
		;
		j -= !IS_ENABLED(CONFIG_ARCH_HAS_PREEMPT_LAZY);

		for (; i < j; i++) {
		if (preempt_dynamic_mode == i)

kernel/sched/sched.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -3633,6 +3633,7 @@ extern int preempt_dynamic_mode;
		extern int sched_dynamic_mode(const char *str);
		extern void sched_dynamic_update(int mode);
		#endif
		extern const char *preempt_modes[];

		#ifdef CONFIG_SCHED_MM_CID