sched/debug: more schedstats fixes

Fixes for two more bugs I found in the schedstats code, in addition to to the other patches I sent out:

https://lkml.kernel.org/r/[email protected]

Josh Poimboeuf (2):
sched/debug: fix 'schedstats=enable' cmdline option sched/debug: fix deadlock when enabling sched events

 include/trace/events/sched.h | 30 +++++++++++++++---------------
 kernel/sched/core.c          | 26 +++++++++++++++++++++-----
 kernel/sched/fair.c          | 29 +++++++----------------------
 3 files changed, 43 insertions(+), 42 deletions(-)

2.4.11

The 'schedstats=enable' option doesn't work, and also produces the following warning during boot:

  WARNING: CPU: 0 PID: 0 at /home/jpoimboe/git/linux/kernel/jump_label.c:61 static_key_slow_inc+0x8c/0xa0
  static_key_slow_inc used before call to jump_label_init
  Modules linked in:
  CPU: 0 PID: 0 Comm: swapper Not tainted 4.7.0-rc1+ #25
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.8.1-20150318_183358- 04/01/2014
   0000000000000086 3ae3475a4bea95d4 ffffffff81e03da8 ffffffff8143fc83
   ffffffff81e03df8 0000000000000000 ffffffff81e03de8 ffffffff810b1ffb
   0000003d00000096 ffffffff823514d0 ffff88007ff197c8 0000000000000000
  Call Trace:
   [<ffffffff8143fc83>] dump_stack+0x85/0xc2
   [<ffffffff810b1ffb>] __warn+0xcb/0xf0
   [<ffffffff810b207f>] warn_slowpath_fmt+0x5f/0x80
   [<ffffffff811e9c0c>] static_key_slow_inc+0x8c/0xa0
   [<ffffffff810e07c6>] static_key_enable+0x16/0x40
   [<ffffffff8216d633>] setup_schedstats+0x29/0x94
   [<ffffffff82148a05>] unknown_bootoption+0x89/0x191
   [<ffffffff810d8617>] parse_args+0x297/0x4b0
   [<ffffffff82148d61>] start_kernel+0x1d8/0x4a9
   [<ffffffff8214897c>] ? set_init_arg+0x55/0x55
   [<ffffffff82148120>] ? early_idt_handler_array+0x120/0x120
   [<ffffffff821482db>] x86_64_start_reservations+0x2f/0x31
   [<ffffffff82148427>] x86_64_start_kernel+0x14a/0x16d

The problem is that it tries to update the 'sched_schedstats' static key before jump labels have been initialized.

Changing jump_label_init() to be called earlier before parse_early_param() wouldn't fix it: it would still fail trying to poke_text() because mm isn't yet initialized.

Instead, just create a temporary '__sched_schedstats' variable which can be copied to the static key later during sched_init() after jump labels have been initialized.

---
 kernel/sched/core.c | 26 +++++++++++++++++++++-----
 1 file changed, 21 insertions(+), 5 deletions(-)

index 7f2cae4..385c947 100644
@@ -2253,9 +2253,11 @@ int sysctl_numa_balancing(struct ctl_table *table, int write,
 #endif
 #endif
 
+#ifdef CONFIG_SCHEDSTATS
+
 DEFINE_STATIC_KEY_FALSE(sched_schedstats);
+static bool __initdata __sched_schedstats = false;
 
-#ifdef CONFIG_SCHEDSTATS
 static void set_schedstats(bool enabled)
 {
 	if (enabled)

@@ -2278,11 +2280,16 @@ static int __init setup_schedstats(char *str)
 	if (!str)
 		goto out;
 
+	/*
+	 * This code is called before jump labels have been set up, so we can't
+	 * change the static branch directly just yet.  Instead set a temporary
+	 * variable so init_schedstats() can do it later.
+	 */
 	if (!strcmp(str, "enable")) {
-		set_schedstats(true);
+		__sched_schedstats = true;
 		ret = 1;
 	} else if (!strcmp(str, "disable")) {
-		set_schedstats(false);
+		__sched_schedstats = false;
 		ret = 1;
 	}
 out:

@@ -2293,6 +2300,11 @@ out:
 }
 __setup("schedstats=", setup_schedstats);
 
+static void __init init_schedstats(void)
+{
+	set_schedstats(__sched_schedstats);
+}
+
 #ifdef CONFIG_PROC_SYSCTL
 int sysctl_schedstats(struct ctl_table *table, int write,
 			 void __user *buffer, size_t *lenp, loff_t *ppos)

@@ -2313,8 +2325,10 @@ int sysctl_schedstats(struct ctl_table *table, int write,
 		set_schedstats(state);
 	return err;
 }
-#endif
-#endif
+#endif /* CONFIG_PROC_SYSCTL */
+#else  /* !CONFIG_SCHEDSTATS */
+static inline void init_schedstats(void) {}
+#endif /* CONFIG_SCHEDSTATS */
 
 /*
  * fork()/clone()-time setup:

@@ -7487,6 +7501,8 @@ void __init sched_init(void)
 #endif
 	init_sched_fair_class();
 
+	init_schedstats();
+
 	scheduler_running = 1;
 }
 

2.4.11

Odd, I thought that particular one had been addressed. At least I saw errors like that in other areas before and they got fixed later by ftrace.

Mel Gorman
SUSE Labs

When enabling sched trace events via:

echo 1 > /sys/kernel/debug/tracing/events/sched/enable

I see a hang, with the following BUG in the printk buffer:

  BUG: spinlock recursion on CPU#1, swapper/1/0
   lock: 0xffff88007d5d8c00, .magic: dead4ead, .owner: swapper/1/0, .owner_cpu: 1
  CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.7.0-rc2+ #1
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.8.1-20150318_183358- 04/01/2014
   0000000000000086 3a8e170cf69ad23f ffff88007d403898 ffffffff8143d663
   ffff88007c535480 ffff88007d5d8c00 ffff88007d4038b8 ffffffff81115948
   ffff88007d5d8c00 ffff88007d5d8c00 ffff88007d4038e8 ffffffff81115aea
  Call Trace:
   <IRQ>  [<ffffffff8143d663>] dump_stack+0x85/0xc2
   [<ffffffff81115948>] spin_dump+0x78/0xc0
   [<ffffffff81115aea>] do_raw_spin_lock+0x11a/0x150
   [<ffffffff81891471>] _raw_spin_lock+0x61/0x80
   [<ffffffff810e5466>] ? try_to_wake_up+0x256/0x4e0
   [<ffffffff810e5466>] try_to_wake_up+0x256/0x4e0
   [<ffffffff81891a0a>] ? _raw_spin_unlock_irqrestore+0x4a/0x80
   [<ffffffff810e5705>] wake_up_process+0x15/0x20
   [<ffffffff810cebb4>] insert_work+0x84/0xc0
   [<ffffffff810ced7f>] __queue_work+0x18f/0x660
   [<ffffffff810cf9a6>] queue_work_on+0x46/0x90
   [<ffffffffa00cd95b>] drm_fb_helper_dirty.isra.11+0xcb/0xe0 [drm_kms_helper]
   [<ffffffffa00cdac0>] drm_fb_helper_sys_imageblit+0x30/0x40 [drm_kms_helper]
   [<ffffffff814babcd>] soft_cursor+0x1ad/0x230
   [<ffffffff814ba379>] bit_cursor+0x649/0x680
   [<ffffffff814b9d30>] ? update_attr.isra.2+0x90/0x90
   [<ffffffff814b5e6a>] fbcon_cursor+0x14a/0x1c0
   [<ffffffff81555ef8>] hide_cursor+0x28/0x90
   [<ffffffff81558b6f>] vt_console_print+0x3bf/0x3f0
   [<ffffffff81122c63>] call_console_drivers.constprop.24+0x183/0x200
   [<ffffffff811241f4>] console_unlock+0x3d4/0x610
   [<ffffffff811247f5>] vprintk_emit+0x3c5/0x610
   [<ffffffff81124bc9>] vprintk_default+0x29/0x40
   [<ffffffff811e965b>] printk+0x57/0x73
   [<ffffffff810f7a9e>] enqueue_entity+0xc2e/0xc70
   [<ffffffff810f7b39>] enqueue_task_fair+0x59/0xab0
   [<ffffffff8106dcd9>] ? kvm_sched_clock_read+0x9/0x20
   [<ffffffff8103fb39>] ? sched_clock+0x9/0x10
   [<ffffffff810e3fcc>] activate_task+0x5c/0xa0
   [<ffffffff810e4514>] ttwu_do_activate+0x54/0xb0
   [<ffffffff810e5cea>] sched_ttwu_pending+0x7a/0xb0
   [<ffffffff810e5e51>] scheduler_ipi+0x61/0x170
   [<ffffffff81059e7f>] smp_trace_reschedule_interrupt+0x4f/0x2a0
   [<ffffffff81893ba6>] trace_reschedule_interrupt+0x96/0xa0
   <EOI>  [<ffffffff8106e0d6>] ? native_safe_halt+0x6/0x10
   [<ffffffff8110fb1d>] ? trace_hardirqs_on+0xd/0x10
   [<ffffffff81040ac0>] default_idle+0x20/0x1a0
   [<ffffffff8104147f>] arch_cpu_idle+0xf/0x20
   [<ffffffff81102f8f>] default_idle_call+0x2f/0x50
   [<ffffffff8110332e>] cpu_startup_entry+0x37e/0x450
   [<ffffffff8105af70>] start_secondary+0x160/0x1a0

The bug is caused by a deadlock where the task is trying to grab the rq lock twice because printk()'s aren't safe in sched code.

The offending printk() is the pr_warn_once() in check_schedstat_required() which warns the user that their 'trace_sched_stat_*' tracepoints aren't going to work because schedstats is disabled:

	/* Force schedstat enabled if a dependent tracepoint is active */
	if (trace_sched_stat_wait_enabled()    ||
			trace_sched_stat_sleep_enabled()   ||
			trace_sched_stat_iowait_enabled()  ||
			trace_sched_stat_blocked_enabled() ||
			trace_sched_stat_runtime_enabled())  {
		pr_warn_once("Scheduler tracepoints stat_sleep, stat_iowait, "
			     "stat_blocked and stat_runtime require the "
			     "kernel parameter schedstats=enabled or "
			     "kernel.sched_schedstats=1\n");
	}

In addition to the deadlock, I think this code has other issues:

1. Instead of just warning and allowing the tracepoints to be broken, I'd argue that it would be better to make them work by forcing schedstats enabled and printing a warning about that, which is what's already being done in other similar cases (latencytop and profiling). Otherwise things like "perf sched record" won't have the intended effect. In fact the comment in the above code snippet seems to agree with me, so maybe that was the original intent.

2. It's called in the scheduler hot path from enqueue_entity().

So change the warning to a call to force_schedstat_enabled(), and do it only when the tracepoint gets enabled.

---
 include/trace/events/sched.h | 30 +++++++++++++++---------------
 kernel/sched/fair.c          | 29 +++++++----------------------
 2 files changed, 22 insertions(+), 37 deletions(-)

index 9b90c57..ce12e93 100644
@@ -341,37 +341,36 @@ DECLARE_EVENT_CLASS(sched_stat_template,
 			(unsigned long long)__entry->delay)
 );
 
+extern void trace_sched_stat_register(void);
+
+#define DEFINE_SCHED_STAT_EVENT(name)					\
+	DEFINE_EVENT_FN(sched_stat_template, name,			\
+			TP_PROTO(struct task_struct *tsk, u64 delay),	\
+			TP_ARGS(tsk, delay),				\
+			trace_sched_stat_register, NULL)
 
 /*
  * Tracepoint for accounting wait time (time the task is runnable
  * but not actually running due to scheduler contention).
  */
-DEFINE_EVENT(sched_stat_template, sched_stat_wait,
-	     TP_PROTO(struct task_struct *tsk, u64 delay),
-	     TP_ARGS(tsk, delay));
+DEFINE_SCHED_STAT_EVENT(sched_stat_wait);
 
 /*
  * Tracepoint for accounting sleep time (time the task is not runnable,
  * including iowait, see below).
  */
-DEFINE_EVENT(sched_stat_template, sched_stat_sleep,
-	     TP_PROTO(struct task_struct *tsk, u64 delay),
-	     TP_ARGS(tsk, delay));
+DEFINE_SCHED_STAT_EVENT(sched_stat_sleep);
 
 /*
  * Tracepoint for accounting iowait time (time the task is not runnable
  * due to waiting on IO to complete).
  */
-DEFINE_EVENT(sched_stat_template, sched_stat_iowait,
-	     TP_PROTO(struct task_struct *tsk, u64 delay),
-	     TP_ARGS(tsk, delay));
+DEFINE_SCHED_STAT_EVENT(sched_stat_iowait);
 
 /*
  * Tracepoint for accounting blocked time (time the task is in uninterruptible).
  */
-DEFINE_EVENT(sched_stat_template, sched_stat_blocked,
-	     TP_PROTO(struct task_struct *tsk, u64 delay),
-	     TP_ARGS(tsk, delay));
+DEFINE_SCHED_STAT_EVENT(sched_stat_blocked);
 
 /*
  * Tracepoint for accounting runtime (time the task is executing

@@ -403,9 +402,10 @@ DECLARE_EVENT_CLASS(sched_stat_runtime,
 			(unsigned long long)__entry->vruntime)
 );
 
-DEFINE_EVENT(sched_stat_runtime, sched_stat_runtime,
-	     TP_PROTO(struct task_struct *tsk, u64 runtime, u64 vruntime),
-	     TP_ARGS(tsk, runtime, vruntime));
+DEFINE_EVENT_FN(sched_stat_runtime, sched_stat_runtime,
+		TP_PROTO(struct task_struct *tsk, u64 runtime, u64 vruntime),
+		TP_ARGS(tsk, runtime, vruntime),
+		trace_sched_stat_register, NULL);
 
 /*
  * Tracepoint for showing priority inheritance modifying a tasks

index c6dd8ba..b0cbcc0 100644
@@ -789,6 +789,13 @@ static void update_curr_fair(struct rq *rq)
 	update_curr(cfs_rq_of(&rq->curr->se));
 }
 
+void trace_sched_stat_register(void)
+{
+#ifdef CONFIG_SCHEDSTATS
+	force_schedstat_enabled();
+#endif
+}
+
 #ifdef CONFIG_SCHEDSTATS
 static inline void
 update_stats_wait_start(struct cfs_rq *cfs_rq, struct sched_entity *se)

@@ -3208,27 +3215,6 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
 
 static void check_enqueue_throttle(struct cfs_rq *cfs_rq);
 
-static inline void check_schedstat_required(void)
-{
-#ifdef CONFIG_SCHEDSTATS
-	if (schedstat_enabled())
-		return;
-
-	/* Force schedstat enabled if a dependent tracepoint is active */
-	if (trace_sched_stat_wait_enabled()    ||
-			trace_sched_stat_sleep_enabled()   ||
-			trace_sched_stat_iowait_enabled()  ||
-			trace_sched_stat_blocked_enabled() ||
-			trace_sched_stat_runtime_enabled())  {
-		pr_warn_once("Scheduler tracepoints stat_sleep, stat_iowait, "
-			     "stat_blocked and stat_runtime require the "
-			     "kernel parameter schedstats=enabled or "
-			     "kernel.sched_schedstats=1\n");
-	}
-#endif
-}
-
-
 /*
  * MIGRATION
  *

@@ -3293,7 +3279,6 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 			enqueue_sleeper(cfs_rq, se);
 	}
 
-	check_schedstat_required();
 	if (schedstat_enabled()) {
 		update_stats_enqueue(cfs_rq, se);
 		check_spread(cfs_rq, se);

2.4.11

Forcing them enabled doesn't make them useful per se; some of these tracepoint rely on previously recoded state, which now wasn't previously recorded because back then it was disabled.

But yes, maybe this is better...

Ah, that's true. Though this issue exists regardless of whether we force enable schedstats or the user manually enables them. It's a side effect of allowing schedstats to be enabled after booting.

Josh

This is unfortunately true. It means there is a window when the data is unreliable after the stats are enabled. It is assumed that either someone enabling the stats knows that or is willing to enable them on the command line.

Mel Gorman
SUSE Labs

Yes, agreed.

Looking at how we could do that:

- The sched_stat_wait tracepoint is dependent on the wait_start schedstat.

- The sched_stat_sleep tracepoint is dependent on the sleep_start schedstat.

- The sched_stat_iowait and sched_stat_blocked tracepoints are dependent on the block_start schedstat.

We could move those three schedstats values out of sched_statistics and into sched_entity, and then always update them regardless of CONFIG_SCHEDSTATS.

That would ensure these tracepoints always work. But then again it would add a little bit of runtime overhead.

I don't have a strong opinion either way. Thoughts?

Josh

Can we maintain wait_start unconditionally? Having config dependent tracepoints sucks.

Ditto.

Ditto.

Please do a before/after 'size vmlinux' dump of a defconfig build with tracing disabled. I.e. how much code overhead does the updating of those variables add?

Thanks,

Ingo

When trying to decouple the sched_stat_* tracepoints from schedstats, I see a significant (~12%) performance degradation in pipetest for the CONFIG_SCHEDSTATS case. Still looking into it...

In the meantime, here's a straightforward fix for the deadlock.

---

From: Josh Poimboeuf <[email protected]>
Subject: [PATCH v2] sched/debug: fix deadlock when enabling sched events

I see a hang when enabling sched events:

echo 1 > /sys/kernel/debug/tracing/events/sched/enable

The printk buffer shows:

  BUG: spinlock recursion on CPU#1, swapper/1/0
   lock: 0xffff88007d5d8c00, .magic: dead4ead, .owner: swapper/1/0, .owner_cpu: 1
  CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.7.0-rc2+ #1
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.8.1-20150318_183358- 04/01/2014
   0000000000000086 3a8e170cf69ad23f ffff88007d403898 ffffffff8143d663
   ffff88007c535480 ffff88007d5d8c00 ffff88007d4038b8 ffffffff81115948
   ffff88007d5d8c00 ffff88007d5d8c00 ffff88007d4038e8 ffffffff81115aea
  Call Trace:
   <IRQ>  [<ffffffff8143d663>] dump_stack+0x85/0xc2
   [<ffffffff81115948>] spin_dump+0x78/0xc0
   [<ffffffff81115aea>] do_raw_spin_lock+0x11a/0x150
   [<ffffffff81891471>] _raw_spin_lock+0x61/0x80
   [<ffffffff810e5466>] ? try_to_wake_up+0x256/0x4e0
   [<ffffffff810e5466>] try_to_wake_up+0x256/0x4e0
   [<ffffffff81891a0a>] ? _raw_spin_unlock_irqrestore+0x4a/0x80
   [<ffffffff810e5705>] wake_up_process+0x15/0x20
   [<ffffffff810cebb4>] insert_work+0x84/0xc0
   [<ffffffff810ced7f>] __queue_work+0x18f/0x660
   [<ffffffff810cf9a6>] queue_work_on+0x46/0x90
   [<ffffffffa00cd95b>] drm_fb_helper_dirty.isra.11+0xcb/0xe0 [drm_kms_helper]
   [<ffffffffa00cdac0>] drm_fb_helper_sys_imageblit+0x30/0x40 [drm_kms_helper]
   [<ffffffff814babcd>] soft_cursor+0x1ad/0x230
   [<ffffffff814ba379>] bit_cursor+0x649/0x680
   [<ffffffff814b9d30>] ? update_attr.isra.2+0x90/0x90
   [<ffffffff814b5e6a>] fbcon_cursor+0x14a/0x1c0
   [<ffffffff81555ef8>] hide_cursor+0x28/0x90
   [<ffffffff81558b6f>] vt_console_print+0x3bf/0x3f0
   [<ffffffff81122c63>] call_console_drivers.constprop.24+0x183/0x200
   [<ffffffff811241f4>] console_unlock+0x3d4/0x610
   [<ffffffff811247f5>] vprintk_emit+0x3c5/0x610
   [<ffffffff81124bc9>] vprintk_default+0x29/0x40
   [<ffffffff811e965b>] printk+0x57/0x73
   [<ffffffff810f7a9e>] enqueue_entity+0xc2e/0xc70
   [<ffffffff810f7b39>] enqueue_task_fair+0x59/0xab0
   [<ffffffff8106dcd9>] ? kvm_sched_clock_read+0x9/0x20
   [<ffffffff8103fb39>] ? sched_clock+0x9/0x10
   [<ffffffff810e3fcc>] activate_task+0x5c/0xa0
   [<ffffffff810e4514>] ttwu_do_activate+0x54/0xb0
   [<ffffffff810e5cea>] sched_ttwu_pending+0x7a/0xb0
   [<ffffffff810e5e51>] scheduler_ipi+0x61/0x170
   [<ffffffff81059e7f>] smp_trace_reschedule_interrupt+0x4f/0x2a0
   [<ffffffff81893ba6>] trace_reschedule_interrupt+0x96/0xa0
   <EOI>  [<ffffffff8106e0d6>] ? native_safe_halt+0x6/0x10
   [<ffffffff8110fb1d>] ? trace_hardirqs_on+0xd/0x10
   [<ffffffff81040ac0>] default_idle+0x20/0x1a0
   [<ffffffff8104147f>] arch_cpu_idle+0xf/0x20
   [<ffffffff81102f8f>] default_idle_call+0x2f/0x50
   [<ffffffff8110332e>] cpu_startup_entry+0x37e/0x450
   [<ffffffff8105af70>] start_secondary+0x160/0x1a0

Note the hang only occurs when echoing the above from a physical serial console, not from an ssh session.

The bug is caused by a deadlock where the task is trying to grab the rq lock twice because printk()'s aren't safe in sched code.

---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index c6dd8ba..52d9b37 100644
@@ -3220,7 +3220,7 @@ static inline void check_schedstat_required(void)
 			trace_sched_stat_iowait_enabled()  ||
 			trace_sched_stat_blocked_enabled() ||
 			trace_sched_stat_runtime_enabled())  {
-		pr_warn_once("Scheduler tracepoints stat_sleep, stat_iowait, "
+		printk_deferred_once("Scheduler tracepoints stat_sleep, stat_iowait, "
 			     "stat_blocked and stat_runtime require the "
 			     "kernel parameter schedstats=enabled or "
 			     "kernel.sched_schedstats=1\n");

2.4.11

I was certain I had tested this case because I was monitoring to make sure the stats were not updating until the tuning knob was used. It was because the context was unsafe that any printing happened because at the time trying to enable stats in that context blew up.

I like it! I wasn't aware there was an option to have a registration callback.

Mel Gorman
SUSE Labs