Re: [RFC PATCH 6/8] preempt/dynamic: Provide preempt_schedule[_notrace]() static calls

[Peter Zijlstra]

On Fri, Jan 22, 2021 at 05:57:53PM +0100, Ard Biesheuvel wrote:
> On Fri, 22 Jan 2021 at 17:53, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Mon, Jan 18, 2021 at 03:12:21PM +0100, Frederic Weisbecker wrote:
> > > +#ifdef CONFIG_PREEMPT_DYNAMIC
> > > +DEFINE_STATIC_CALL(preempt_schedule, __preempt_schedule_func());
> > > +EXPORT_STATIC_CALL(preempt_schedule);
> > > +#endif
> >
> > > +#ifdef CONFIG_PREEMPT_DYNAMIC
> > > +DEFINE_STATIC_CALL(preempt_schedule_notrace, __preempt_schedule_notrace_func());
> > > +EXPORT_STATIC_CALL(preempt_schedule_notrace);
> > > +#endif
> >
> > So one of the things I hates most of this is that is allows 'random'
> > modules to hijack the preemption by rewriting these callsites. Once you
> > export the key, we've lost.
> >
> 
> Are these supposed to be switchable at any time? Or only at boot? In
> the latter case, can't we drop the associated data structure in
> __ro_after_init so it becomes R/O when booting completes?

Doesn't work, loading modules modifies the key -- we recently had
someone complain about that for jump_label.

And also, I have this patch...

---
Subject: sched: Add /debug/sched_preempt
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Fri Jan 22 13:01:58 CET 2021

Add a debugfs file to muck about with the preempt mode at runtime.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/sched/core.c |  135 ++++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 126 insertions(+), 9 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5362,37 +5362,154 @@ EXPORT_STATIC_CALL(preempt_schedule_notr
  *   preempt_schedule_notrace   <- preempt_schedule_notrace
  *   irqentry_exit_cond_resched <- irqentry_exit_cond_resched
  */
-static int __init setup_preempt_mode(char *str)
+
+enum {
+	preempt_dynamic_none = 0,
+	preempt_dynamic_voluntary,
+	preempt_dynamic_full,
+};
+
+static int preempt_dynamic_mode = preempt_dynamic_full;
+
+static int sched_dynamic_mode(const char *str)
+{
+	if (!strcmp(str, "none"))
+		return 0;
+
+	if (!strcmp(str, "voluntary"))
+		return 1;
+
+	if (!strcmp(str, "full"))
+		return 2;
+
+	return -1;
+}
+
+static void sched_dynamic_update(int mode)
 {
-	if (!strcmp(str, "none")) {
+	/*
+	 * Avoid {NONE,VOLUNTARY} -> FULL transitions from ever ending up in
+	 * the ZERO state, which is invalid.
+	 */
+	static_call_update(cond_resched, __cond_resched);
+	static_call_update(might_resched, __cond_resched);
+	static_call_update(preempt_schedule, __preempt_schedule_func());
+	static_call_update(preempt_schedule_notrace, __preempt_schedule_notrace_func());
+	static_call_update(irqentry_exit_cond_resched, irqentry_exit_cond_resched);
+
+	switch (mode) {
+	case preempt_dynamic_none:
 		static_call_update(cond_resched, __cond_resched);
 		static_call_update(might_resched, (typeof(&__cond_resched)) __static_call_return0);
 		static_call_update(preempt_schedule, (typeof(&preempt_schedule)) NULL);
 		static_call_update(preempt_schedule_notrace, (typeof(&preempt_schedule_notrace)) NULL);
 		static_call_update(irqentry_exit_cond_resched, (typeof(&irqentry_exit_cond_resched)) NULL);
-		pr_info("Dynamic Preempt: %s\n", str);
-	} else if (!strcmp(str, "voluntary")) {
+		pr_info("Dynamic Preempt: none\n");
+		break;
+
+	case preempt_dynamic_voluntary:
 		static_call_update(cond_resched, __cond_resched);
 		static_call_update(might_resched, __cond_resched);
 		static_call_update(preempt_schedule, (typeof(&preempt_schedule)) NULL);
 		static_call_update(preempt_schedule_notrace, (typeof(&preempt_schedule_notrace)) NULL);
 		static_call_update(irqentry_exit_cond_resched, (typeof(&irqentry_exit_cond_resched)) NULL);
-		pr_info("Dynamic Preempt: %s\n", str);
-	} else if (!strcmp(str, "full")) {
+		pr_info("Dynamic Preempt: voluntary\n");
+		break;
+
+	case preempt_dynamic_full:
 		static_call_update(cond_resched, (typeof(&__cond_resched)) __static_call_return0);
 		static_call_update(might_resched, (typeof(&__cond_resched)) __static_call_return0);
 		static_call_update(preempt_schedule, __preempt_schedule_func());
 		static_call_update(preempt_schedule_notrace, __preempt_schedule_notrace_func());
 		static_call_update(irqentry_exit_cond_resched, irqentry_exit_cond_resched);
-		pr_info("Dynamic Preempt: %s\n", str);
-	} else {
-		pr_warn("Dynamic Preempt: Unsupported preempt mode %s, default to full\n", str);
+		pr_info("Dynamic Preempt: full\n");
+		break;
+	}
+
+	preempt_dynamic_mode = mode;
+}
+
+static int __init setup_preempt_mode(char *str)
+{
+	int mode = sched_dynamic_mode(str);
+	if (mode < 0) {
+		pr_warn("Dynamic Preempt: unsupported mode: %s\n", str);
 		return 1;
 	}
+
+	sched_dynamic_update(mode);
 	return 0;
 }
 __setup("preempt=", setup_preempt_mode);
 
+#ifdef CONFIG_SCHED_DEBUG
+
+static ssize_t sched_dynamic_write(struct file *filp, const char __user *ubuf,
+				   size_t cnt, loff_t *ppos)
+{
+	char buf[16];
+	int mode;
+
+	if (cnt > 15)
+		cnt = 15;
+
+	if (copy_from_user(&buf, ubuf, cnt))
+		return -EFAULT;
+
+	buf[cnt] = 0;
+	mode = sched_dynamic_mode(strstrip(buf));
+	if (mode < 0)
+		return mode;
+
+	sched_dynamic_update(mode);
+
+	*ppos += cnt;
+
+	return cnt;
+}
+
+static int sched_dynamic_show(struct seq_file *m, void *v)
+{
+	static const char * preempt_modes[] = {
+		"none", "voluntary", "full"
+	};
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(preempt_modes); i++) {
+		if (preempt_dynamic_mode == i)
+			seq_puts(m, "(");
+		seq_puts(m, preempt_modes[i]);
+		if (preempt_dynamic_mode == i)
+			seq_puts(m, ")");
+
+		seq_puts(m, " ");
+	}
+
+	seq_puts(m, "\n");
+	return 0;
+}
+
+static int sched_dynamic_open(struct inode *inode, struct file *filp)
+{
+	return single_open(filp, sched_dynamic_show, NULL);
+}
+
+static const struct file_operations sched_dynamic_fops = {
+	.open		= sched_dynamic_open,
+	.write		= sched_dynamic_write,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= single_release,
+};
+
+static __init int sched_init_debug_dynamic(void)
+{
+	debugfs_create_file("sched_preempt", 0644, NULL, NULL, &sched_dynamic_fops);
+	return 0;
+}
+late_initcall(sched_init_debug_dynamic);
+
+#endif /* CONFIG_SCHED_DEBUG */
 #endif /* CONFIG_PREEMPT_DYNAMIC */