delayacct: Default disabled

Assuming this stuff isn't actually used much; disable it by default
and avoid allocating and tracking the task_delay_info structure.

taskstats is changed to still report the regular sched and sched_info
and only skip the missing task_delay_info fields instead of not
reporting anything.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20210505111525.308018373@infradead.org

Documentation/accounting/delay-accounting.rst

@@ -69,13 +69,13 @@ Compile the kernel with::
 	CONFIG_TASK_DELAY_ACCT=y
 	CONFIG_TASKSTATS=y
 
-Delay accounting is enabled by default at boot up.
-To disable, add::
+Delay accounting is disabled by default at boot up.
+To enable, add::
 
-   nodelayacct
+   delayacct
 
 to the kernel boot options. The rest of the instructions
-below assume this has not been done.
+below assume this has been done.
 
 After the system has booted up, use a utility
 similar to  getdelays.c to access the delays

Documentation/admin-guide/kernel-parameters.txt

@@ -3244,7 +3244,7 @@
 
 	noclflush	[BUGS=X86] Don't use the CLFLUSH instruction
 
-	nodelayacct	[KNL] Disable per-task delay accounting
+	delayacct	[KNL] Enable per-task delay accounting
 
 	nodsp		[SH] Disable hardware DSP at boot time.
 

include/linux/delayacct.h

@@ -61,15 +61,15 @@ struct task_delay_info {
 #include <linux/jump_label.h>
 
 #ifdef CONFIG_TASK_DELAY_ACCT
-DECLARE_STATIC_KEY_TRUE(delayacct_key);
+DECLARE_STATIC_KEY_FALSE(delayacct_key);
 extern int delayacct_on;	/* Delay accounting turned on/off */
 extern struct kmem_cache *delayacct_cache;
 extern void delayacct_init(void);
 extern void __delayacct_tsk_init(struct task_struct *);
 extern void __delayacct_tsk_exit(struct task_struct *);
 extern void __delayacct_blkio_start(void);
 extern void __delayacct_blkio_end(struct task_struct *);
-extern int __delayacct_add_tsk(struct taskstats *, struct task_struct *);
+extern int delayacct_add_tsk(struct taskstats *, struct task_struct *);
 extern __u64 __delayacct_blkio_ticks(struct task_struct *);
 extern void __delayacct_freepages_start(void);
 extern void __delayacct_freepages_end(void);
@@ -116,7 +116,7 @@ static inline void delayacct_tsk_free(struct task_struct *tsk)
 
 static inline void delayacct_blkio_start(void)
 {
-	if (!static_branch_likely(&delayacct_key))
+	if (!static_branch_unlikely(&delayacct_key))
 		return;
 
 	delayacct_set_flag(current, DELAYACCT_PF_BLKIO);
@@ -126,22 +126,14 @@ static inline void delayacct_blkio_start(void)
 
 static inline void delayacct_blkio_end(struct task_struct *p)
 {
-	if (!static_branch_likely(&delayacct_key))
+	if (!static_branch_unlikely(&delayacct_key))
 		return;
 
 	if (p->delays)
 		__delayacct_blkio_end(p);
 	delayacct_clear_flag(p, DELAYACCT_PF_BLKIO);
 }
 
-static inline int delayacct_add_tsk(struct taskstats *d,
-					struct task_struct *tsk)
-{
-	if (!delayacct_on || !tsk->delays)
-		return 0;
-	return __delayacct_add_tsk(d, tsk);
-}
-
 static inline __u64 delayacct_blkio_ticks(struct task_struct *tsk)
 {
 	if (tsk->delays)

kernel/delayacct.c

@@ -14,23 +14,23 @@
 #include <linux/delayacct.h>
 #include <linux/module.h>
 
-DEFINE_STATIC_KEY_TRUE(delayacct_key);
-int delayacct_on __read_mostly = 1;	/* Delay accounting turned on/off */
+DEFINE_STATIC_KEY_FALSE(delayacct_key);
+int delayacct_on __read_mostly;	/* Delay accounting turned on/off */
 struct kmem_cache *delayacct_cache;
 
-static int __init delayacct_setup_disable(char *str)
+static int __init delayacct_setup_enable(char *str)
 {
-	delayacct_on = 0;
+	delayacct_on = 1;
 	return 1;
 }
-__setup("nodelayacct", delayacct_setup_disable);
+__setup("delayacct", delayacct_setup_enable);
 
 void delayacct_init(void)
 {
 	delayacct_cache = KMEM_CACHE(task_delay_info, SLAB_PANIC|SLAB_ACCOUNT);
 	delayacct_tsk_init(&init_task);
-	if (!delayacct_on)
-		static_branch_disable(&delayacct_key);
+	if (delayacct_on)
+		static_branch_enable(&delayacct_key);
 }
 
 void __delayacct_tsk_init(struct task_struct *tsk)
@@ -83,7 +83,7 @@ void __delayacct_blkio_end(struct task_struct *p)
 	delayacct_end(&delays->lock, &delays->blkio_start, total, count);
 }
 
-int __delayacct_add_tsk(struct taskstats *d, struct task_struct *tsk)
+int delayacct_add_tsk(struct taskstats *d, struct task_struct *tsk)
 {
 	u64 utime, stime, stimescaled, utimescaled;
 	unsigned long long t2, t3;
@@ -118,6 +118,9 @@ int __delayacct_add_tsk(struct taskstats *d, struct task_struct *tsk)
 	d->cpu_run_virtual_total =
 		(tmp < (s64)d->cpu_run_virtual_total) ?	0 : tmp;
 
+	if (!tsk->delays)
+		return 0;
+
 	/* zero XXX_total, non-zero XXX_count implies XXX stat overflowed */
 
 	raw_spin_lock_irqsave(&tsk->delays->lock, flags);