sched/debug: Make schedstats a runtime tunable that is disabled by default

schedstats is very useful during debugging and performance tuning but it
incurs overhead to calculate the stats. As such, even though it can be
disabled at build time, it is often enabled as the information is useful.

This patch adds a kernel command-line and sysctl tunable to enable or
disable schedstats on demand (when it's built in). It is disabled
by default as someone who knows they need it can also learn to enable
it when necessary.

The benefits are dependent on how scheduler-intensive the workload is.
If it is then the patch reduces the number of cycles spent calculating
the stats with a small benefit from reducing the cache footprint of the
scheduler.

These measurements were taken from a 48-core 2-socket
machine with Xeon(R) E5-2670 v3 cpus although they were also tested on a
single socket machine 8-core machine with Intel i7-3770 processors.

netperf-tcp
                           4.5.0-rc1             4.5.0-rc1
                             vanilla          nostats-v3r1
Hmean    64         560.45 (  0.00%)      575.98 (  2.77%)
Hmean    128        766.66 (  0.00%)      795.79 (  3.80%)
Hmean    256        950.51 (  0.00%)      981.50 (  3.26%)
Hmean    1024      1433.25 (  0.00%)     1466.51 (  2.32%)
Hmean    2048      2810.54 (  0.00%)     2879.75 (  2.46%)
Hmean    3312      4618.18 (  0.00%)     4682.09 (  1.38%)
Hmean    4096      5306.42 (  0.00%)     5346.39 (  0.75%)
Hmean    8192     10581.44 (  0.00%)    10698.15 (  1.10%)
Hmean    16384    18857.70 (  0.00%)    18937.61 (  0.42%)

Small gains here, UDP_STREAM showed nothing intresting and neither did
the TCP_RR tests. The gains on the 8-core machine were very similar.

tbench4
                                 4.5.0-rc1             4.5.0-rc1
                                   vanilla          nostats-v3r1
Hmean    mb/sec-1         500.85 (  0.00%)      522.43 (  4.31%)
Hmean    mb/sec-2         984.66 (  0.00%)     1018.19 (  3.41%)
Hmean    mb/sec-4        1827.91 (  0.00%)     1847.78 (  1.09%)
Hmean    mb/sec-8        3561.36 (  0.00%)     3611.28 (  1.40%)
Hmean    mb/sec-16       5824.52 (  0.00%)     5929.03 (  1.79%)
Hmean    mb/sec-32      10943.10 (  0.00%)    10802.83 ( -1.28%)
Hmean    mb/sec-64      15950.81 (  0.00%)    16211.31 (  1.63%)
Hmean    mb/sec-128     15302.17 (  0.00%)    15445.11 (  0.93%)
Hmean    mb/sec-256     14866.18 (  0.00%)    15088.73 (  1.50%)
Hmean    mb/sec-512     15223.31 (  0.00%)    15373.69 (  0.99%)
Hmean    mb/sec-1024    14574.25 (  0.00%)    14598.02 (  0.16%)
Hmean    mb/sec-2048    13569.02 (  0.00%)    13733.86 (  1.21%)
Hmean    mb/sec-3072    12865.98 (  0.00%)    13209.23 (  2.67%)

Small gains of 2-4% at low thread counts and otherwise flat.  The
gains on the 8-core machine were slightly different

tbench4 on 8-core i7-3770 single socket machine
Hmean    mb/sec-1        442.59 (  0.00%)      448.73 (  1.39%)
Hmean    mb/sec-2        796.68 (  0.00%)      794.39 ( -0.29%)
Hmean    mb/sec-4       1322.52 (  0.00%)     1343.66 (  1.60%)
Hmean    mb/sec-8       2611.65 (  0.00%)     2694.86 (  3.19%)
Hmean    mb/sec-16      2537.07 (  0.00%)     2609.34 (  2.85%)
Hmean    mb/sec-32      2506.02 (  0.00%)     2578.18 (  2.88%)
Hmean    mb/sec-64      2511.06 (  0.00%)     2569.16 (  2.31%)
Hmean    mb/sec-128     2313.38 (  0.00%)     2395.50 (  3.55%)
Hmean    mb/sec-256     2110.04 (  0.00%)     2177.45 (  3.19%)
Hmean    mb/sec-512     2072.51 (  0.00%)     2053.97 ( -0.89%)

In constract, this shows a relatively steady 2-3% gain at higher thread
counts. Due to the nature of the patch and the type of workload, it's
not a surprise that the result will depend on the CPU used.

hackbench-pipes
                         4.5.0-rc1             4.5.0-rc1
                           vanilla          nostats-v3r1
Amean    1        0.0637 (  0.00%)      0.0660 ( -3.59%)
Amean    4        0.1229 (  0.00%)      0.1181 (  3.84%)
Amean    7        0.1921 (  0.00%)      0.1911 (  0.52%)
Amean    12       0.3117 (  0.00%)      0.2923 (  6.23%)
Amean    21       0.4050 (  0.00%)      0.3899 (  3.74%)
Amean    30       0.4586 (  0.00%)      0.4433 (  3.33%)
Amean    48       0.5910 (  0.00%)      0.5694 (  3.65%)
Amean    79       0.8663 (  0.00%)      0.8626 (  0.43%)
Amean    110      1.1543 (  0.00%)      1.1517 (  0.22%)
Amean    141      1.4457 (  0.00%)      1.4290 (  1.16%)
Amean    172      1.7090 (  0.00%)      1.6924 (  0.97%)
Amean    192      1.9126 (  0.00%)      1.9089 (  0.19%)

Some small gains and losses and while the variance data is not included,
it's close to the noise. The UMA machine did not show anything particularly
different

pipetest
                             4.5.0-rc1             4.5.0-rc1
                               vanilla          nostats-v2r2
Min         Time        4.13 (  0.00%)        3.99 (  3.39%)
1st-qrtle   Time        4.38 (  0.00%)        4.27 (  2.51%)
2nd-qrtle   Time        4.46 (  0.00%)        4.39 (  1.57%)
3rd-qrtle   Time        4.56 (  0.00%)        4.51 (  1.10%)
Max-90%     Time        4.67 (  0.00%)        4.60 (  1.50%)
Max-93%     Time        4.71 (  0.00%)        4.65 (  1.27%)
Max-95%     Time        4.74 (  0.00%)        4.71 (  0.63%)
Max-99%     Time        4.88 (  0.00%)        4.79 (  1.84%)
Max         Time        4.93 (  0.00%)        4.83 (  2.03%)
Mean        Time        4.48 (  0.00%)        4.39 (  1.91%)
Best99%Mean Time        4.47 (  0.00%)        4.39 (  1.91%)
Best95%Mean Time        4.46 (  0.00%)        4.38 (  1.93%)
Best90%Mean Time        4.45 (  0.00%)        4.36 (  1.98%)
Best50%Mean Time        4.36 (  0.00%)        4.25 (  2.49%)
Best10%Mean Time        4.23 (  0.00%)        4.10 (  3.13%)
Best5%Mean  Time        4.19 (  0.00%)        4.06 (  3.20%)
Best1%Mean  Time        4.13 (  0.00%)        4.00 (  3.39%)

Small improvement and similar gains were seen on the UMA machine.

The gain is small but it stands to reason that doing less work in the
scheduler is a good thing. The downside is that the lack of schedstats and
tracepoints may be surprising to experts doing performance analysis until
they find the existence of the schedstats= parameter or schedstats sysctl.
It will be automatically activated for latencytop and sleep profiling to
alleviate the problem. For tracepoints, there is a simple warning as it's
not safe to activate schedstats in the context when it's known the tracepoint
may be wanted but is unavailable.

Signed-off-by: Mel Gorman <[email protected]>
Reviewed-by: Matt Fleming <[email protected]>
Reviewed-by: Srikar Dronamraju <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: gormanm

Documentation/kernel-parameters.txt

@@ -3528,6 +3528,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
 	sched_debug	[KNL] Enables verbose scheduler debug messages.
 
+	schedstats=	[KNL,X86] Enable or disable scheduled statistics.
+			Allowed values are enable and disable. This feature
+			incurs a small amount of overhead in the scheduler
+			but is useful for debugging and performance tuning.
+
 	skew_tick=	[KNL] Offset the periodic timer tick per cpu to mitigate
 			xtime_lock contention on larger systems, and/or RCU lock
 			contention on all systems with CONFIG_MAXSMP set.

Documentation/sysctl/kernel.txt

@@ -760,6 +760,14 @@ rtsig-nr shows the number of RT signals currently queued.
 
 ==============================================================
 
+sched_schedstats:
+
+Enables/disables scheduler statistics. Enabling this feature
+incurs a small amount of overhead in the scheduler but is
+useful for debugging and performance tuning.
+
+==============================================================
+
 sg-big-buff:
 
 This file shows the size of the generic SCSI (sg) buffer.

include/linux/latencytop.h

@@ -37,6 +37,9 @@ account_scheduler_latency(struct task_struct *task, int usecs, int inter)
 
 void clear_all_latency_tracing(struct task_struct *p);
 
+extern int sysctl_latencytop(struct ctl_table *table, int write,
+			void __user *buffer, size_t *lenp, loff_t *ppos);
+
 #else
 
 static inline void

include/linux/sched.h

@@ -920,6 +920,10 @@ static inline int sched_info_on(void)
 #endif
 }
 
+#ifdef CONFIG_SCHEDSTATS
+void force_schedstat_enabled(void);
+#endif
+
 enum cpu_idle_type {
 	CPU_IDLE,
 	CPU_NOT_IDLE,

include/linux/sched/sysctl.h

@@ -95,4 +95,8 @@ extern int sysctl_numa_balancing(struct ctl_table *table, int write,
 				 void __user *buffer, size_t *lenp,
 				 loff_t *ppos);
 
+extern int sysctl_schedstats(struct ctl_table *table, int write,
+				 void __user *buffer, size_t *lenp,
+				 loff_t *ppos);
+
 #endif /* _SCHED_SYSCTL_H */

kernel/latencytop.c

@@ -47,12 +47,12 @@
  * of times)
  */
 
-#include <linux/latencytop.h>
 #include <linux/kallsyms.h>
 #include <linux/seq_file.h>
 #include <linux/notifier.h>
 #include <linux/spinlock.h>
 #include <linux/proc_fs.h>
+#include <linux/latencytop.h>
 #include <linux/export.h>
 #include <linux/sched.h>
 #include <linux/list.h>
@@ -289,4 +289,16 @@ static int __init init_lstats_procfs(void)
 	proc_create("latency_stats", 0644, NULL, &lstats_fops);
 	return 0;
 }
+
+int sysctl_latencytop(struct ctl_table *table, int write,
+			void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+	int err;
+
+	err = proc_dointvec(table, write, buffer, lenp, ppos);
+	if (latencytop_enabled)
+		force_schedstat_enabled();
+
+	return err;
+}
 device_initcall(init_lstats_procfs);

kernel/profile.c

@@ -59,6 +59,7 @@ int profile_setup(char *str)
 
 	if (!strncmp(str, sleepstr, strlen(sleepstr))) {
 #ifdef CONFIG_SCHEDSTATS
+		force_schedstat_enabled();
 		prof_on = SLEEP_PROFILING;
 		if (str[strlen(sleepstr)] == ',')
 			str += strlen(sleepstr) + 1;

kernel/sched/core.c

@@ -2093,7 +2093,8 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 
 	ttwu_queue(p, cpu);
 stat:
-	ttwu_stat(p, cpu, wake_flags);
+	if (schedstat_enabled())
+		ttwu_stat(p, cpu, wake_flags);
 out:
 	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
 
@@ -2141,7 +2142,8 @@ static void try_to_wake_up_local(struct task_struct *p)
 		ttwu_activate(rq, p, ENQUEUE_WAKEUP);
 
 	ttwu_do_wakeup(rq, p, 0);
-	ttwu_stat(p, smp_processor_id(), 0);
+	if (schedstat_enabled())
+		ttwu_stat(p, smp_processor_id(), 0);
 out:
 	raw_spin_unlock(&p->pi_lock);
 }
@@ -2210,6 +2212,7 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
 #endif
 
 #ifdef CONFIG_SCHEDSTATS
+	/* Even if schedstat is disabled, there should not be garbage */
 	memset(&p->se.statistics, 0, sizeof(p->se.statistics));
 #endif
 
@@ -2281,6 +2284,69 @@ int sysctl_numa_balancing(struct ctl_table *table, int write,
 #endif
 #endif
 
+DEFINE_STATIC_KEY_FALSE(sched_schedstats);
+
+#ifdef CONFIG_SCHEDSTATS
+static void set_schedstats(bool enabled)
+{
+	if (enabled)
+		static_branch_enable(&sched_schedstats);
+	else
+		static_branch_disable(&sched_schedstats);
+}
+
+void force_schedstat_enabled(void)
+{
+	if (!schedstat_enabled()) {
+		pr_info("kernel profiling enabled schedstats, disable via kernel.sched_schedstats.\n");
+		static_branch_enable(&sched_schedstats);
+	}
+}
+
+static int __init setup_schedstats(char *str)
+{
+	int ret = 0;
+	if (!str)
+		goto out;
+
+	if (!strcmp(str, "enable")) {
+		set_schedstats(true);
+		ret = 1;
+	} else if (!strcmp(str, "disable")) {
+		set_schedstats(false);
+		ret = 1;
+	}
+out:
+	if (!ret)
+		pr_warn("Unable to parse schedstats=\n");
+
+	return ret;
+}
+__setup("schedstats=", setup_schedstats);
+
+#ifdef CONFIG_PROC_SYSCTL
+int sysctl_schedstats(struct ctl_table *table, int write,
+			 void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+	struct ctl_table t;
+	int err;
+	int state = static_branch_likely(&sched_schedstats);
+
+	if (write && !capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	t = *table;
+	t.data = &state;
+	err = proc_dointvec_minmax(&t, write, buffer, lenp, ppos);
+	if (err < 0)
+		return err;
+	if (write)
+		set_schedstats(state);
+	return err;
+}
+#endif
+#endif
+
 /*
  * fork()/clone()-time setup:
  */

kernel/sched/debug.c

@@ -75,16 +75,18 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group
 	PN(se->vruntime);
 	PN(se->sum_exec_runtime);
 #ifdef CONFIG_SCHEDSTATS
-	PN(se->statistics.wait_start);
-	PN(se->statistics.sleep_start);
-	PN(se->statistics.block_start);
-	PN(se->statistics.sleep_max);
-	PN(se->statistics.block_max);
-	PN(se->statistics.exec_max);
-	PN(se->statistics.slice_max);
-	PN(se->statistics.wait_max);
-	PN(se->statistics.wait_sum);
-	P(se->statistics.wait_count);
+	if (schedstat_enabled()) {
+		PN(se->statistics.wait_start);
+		PN(se->statistics.sleep_start);
+		PN(se->statistics.block_start);
+		PN(se->statistics.sleep_max);
+		PN(se->statistics.block_max);
+		PN(se->statistics.exec_max);
+		PN(se->statistics.slice_max);
+		PN(se->statistics.wait_max);
+		PN(se->statistics.wait_sum);
+		P(se->statistics.wait_count);
+	}
 #endif
 	P(se->load.weight);
 #ifdef CONFIG_SMP
@@ -122,10 +124,12 @@ print_task(struct seq_file *m, struct rq *rq, struct task_struct *p)
 		(long long)(p->nvcsw + p->nivcsw),
 		p->prio);
 #ifdef CONFIG_SCHEDSTATS
-	SEQ_printf(m, "%9Ld.%06ld %9Ld.%06ld %9Ld.%06ld",
-		SPLIT_NS(p->se.statistics.wait_sum),
-		SPLIT_NS(p->se.sum_exec_runtime),
-		SPLIT_NS(p->se.statistics.sum_sleep_runtime));
+	if (schedstat_enabled()) {
+		SEQ_printf(m, "%9Ld.%06ld %9Ld.%06ld %9Ld.%06ld",
+			SPLIT_NS(p->se.statistics.wait_sum),
+			SPLIT_NS(p->se.sum_exec_runtime),
+			SPLIT_NS(p->se.statistics.sum_sleep_runtime));
+	}
 #else
 	SEQ_printf(m, "%9Ld.%06ld %9Ld.%06ld %9Ld.%06ld",
 		0LL, 0L,
@@ -313,17 +317,18 @@ do {									\
 #define P(n) SEQ_printf(m, "  .%-30s: %d\n", #n, rq->n);
 #define P64(n) SEQ_printf(m, "  .%-30s: %Ld\n", #n, rq->n);
 
-	P(yld_count);
-
-	P(sched_count);
-	P(sched_goidle);
 #ifdef CONFIG_SMP
 	P64(avg_idle);
 	P64(max_idle_balance_cost);
 #endif
 
-	P(ttwu_count);
-	P(ttwu_local);
+	if (schedstat_enabled()) {
+		P(yld_count);
+		P(sched_count);
+		P(sched_goidle);
+		P(ttwu_count);
+		P(ttwu_local);
+	}
 
 #undef P
 #undef P64
@@ -569,38 +574,39 @@ void proc_sched_show_task(struct task_struct *p, struct seq_file *m)
 	nr_switches = p->nvcsw + p->nivcsw;
 
 #ifdef CONFIG_SCHEDSTATS
-	PN(se.statistics.sum_sleep_runtime);
-	PN(se.statistics.wait_start);
-	PN(se.statistics.sleep_start);
-	PN(se.statistics.block_start);
-	PN(se.statistics.sleep_max);
-	PN(se.statistics.block_max);
-	PN(se.statistics.exec_max);
-	PN(se.statistics.slice_max);
-	PN(se.statistics.wait_max);
-	PN(se.statistics.wait_sum);
-	P(se.statistics.wait_count);
-	PN(se.statistics.iowait_sum);
-	P(se.statistics.iowait_count);
 	P(se.nr_migrations);
-	P(se.statistics.nr_migrations_cold);
-	P(se.statistics.nr_failed_migrations_affine);
-	P(se.statistics.nr_failed_migrations_running);
-	P(se.statistics.nr_failed_migrations_hot);
-	P(se.statistics.nr_forced_migrations);
-	P(se.statistics.nr_wakeups);
-	P(se.statistics.nr_wakeups_sync);
-	P(se.statistics.nr_wakeups_migrate);
-	P(se.statistics.nr_wakeups_local);
-	P(se.statistics.nr_wakeups_remote);
-	P(se.statistics.nr_wakeups_affine);
-	P(se.statistics.nr_wakeups_affine_attempts);
-	P(se.statistics.nr_wakeups_passive);
-	P(se.statistics.nr_wakeups_idle);
 
-	{
+	if (schedstat_enabled()) {
 		u64 avg_atom, avg_per_cpu;
 
+		PN(se.statistics.sum_sleep_runtime);
+		PN(se.statistics.wait_start);
+		PN(se.statistics.sleep_start);
+		PN(se.statistics.block_start);
+		PN(se.statistics.sleep_max);
+		PN(se.statistics.block_max);
+		PN(se.statistics.exec_max);
+		PN(se.statistics.slice_max);
+		PN(se.statistics.wait_max);
+		PN(se.statistics.wait_sum);
+		P(se.statistics.wait_count);
+		PN(se.statistics.iowait_sum);
+		P(se.statistics.iowait_count);
+		P(se.statistics.nr_migrations_cold);
+		P(se.statistics.nr_failed_migrations_affine);
+		P(se.statistics.nr_failed_migrations_running);
+		P(se.statistics.nr_failed_migrations_hot);
+		P(se.statistics.nr_forced_migrations);
+		P(se.statistics.nr_wakeups);
+		P(se.statistics.nr_wakeups_sync);
+		P(se.statistics.nr_wakeups_migrate);
+		P(se.statistics.nr_wakeups_local);
+		P(se.statistics.nr_wakeups_remote);
+		P(se.statistics.nr_wakeups_affine);
+		P(se.statistics.nr_wakeups_affine_attempts);
+		P(se.statistics.nr_wakeups_passive);
+		P(se.statistics.nr_wakeups_idle);
+
 		avg_atom = p->se.sum_exec_runtime;
 		if (nr_switches)
 			avg_atom = div64_ul(avg_atom, nr_switches);