The content reuse need include the original link: http://oliveryang.net
- 1. What is the SCHEDSTATS?
- 2. How could we access SCHEDSTATS counters?
- 3. SCHEDSTATS proc files use cases
- 4. SCHEDSTATS source files
- 5. Related Readings
1. What is the SCHEDSTATS?
SCHEDSTATS is a kernel debug feature which allows scheduler exports its pre-defined performance counters to user space. We can do following things by collecting and analyzing these perf counters,
- Debug or tune scheduler
- Debug or tune specific application or benchmark from scheduling perspective
2. How could we access SCHEDSTATS counters?
When SCHEDSTATS is enabled, scheduler statistics could be accessed by following ways,
-
Three proc files exported by SCHEDSTATS code
/proc/schedstat, /proc/<pid>/schedstat, /proc/<pid>/sched
Documentation/scheduler/sched-stats.txt file has the full description for file format. We can write user space tools to read and process the proc files.
-
pre-defined kernel trace points
Kernel trace points could be used by dynamic tracing tools, such as systemtap, perf. So far, in Linux 4.1, there are 4 sched_stat_* trace points defined by SCHEDSTATS code, there are 4 sched_stat_* trace points defined by SCHEDSTATS code.
# perf list | grep sched_stat_ sched:sched_stat_wait [Tracepoint event] sched:sched_stat_sleep [Tracepoint event] sched:sched_stat_iowait [Tracepoint event] sched:sched_stat_blocked [Tracepoint event] sched:sched_stat_runtime [Tracepoint event] >>> Not a SCHEDSTAT trace pointLinux perf tool, record, report, script sub-commands could be used for getting system wide or per-task statistics.
-
sleep profiler when SCHEDSTATS is enabled
This needs readprofile command installed in user space. The usage of readprofile could be found from Documentation/basic_profiling.txt. To enable kernel profiler, please refer to Documentation/kernel-parameters.txt. This way is a legacy way and could be replaced by following trace point in latest kernel,
# perf list | grep sched_stat_blocked sched:sched_stat_blocked [Tracepoint event] # perf record -e sched:sched_stat_blocked -a -g sleep 5 # perf script
3. SCHEDSTATS proc files use cases
3.1 System wide statistic
This includes per-cpu(run queue) or per-sched-domain statistics.
/proc/schedstat
Implements in scheduler core, which is the common layer for all scheduling classes.
The CPU statistics in /proc/schedstat file is defined as members of struct rq in kernel/sched.c,
struct rq {
[...snipped...]
#ifdef CONFIG_SCHEDSTATS
/* latency stats */
struct sched_info rq_sched_info;
unsigned long long rq_cpu_time;
/* could above be rq->cfs_rq.exec_clock + rq->rt_rq.rt_runtime ? */
/* sys_sched_yield() stats */
unsigned int yld_count;
/* schedule() stats */
unsigned int sched_switch;
unsigned int sched_count;
unsigned int sched_goidle;
/* try_to_wake_up() stats */
unsigned int ttwu_count;
unsigned int ttwu_local;
#endif
[...snipped...]
};
The Domain statistics in /proc/schedstat file is defined as members of struct sched_domain
in include/linux/sched.h,
struct sched_domain {
[...snipped...]
#ifdef CONFIG_SCHEDSTATS
/* load_balance() stats */
unsigned int lb_count[CPU_MAX_IDLE_TYPES];
unsigned int lb_failed[CPU_MAX_IDLE_TYPES];
unsigned int lb_balanced[CPU_MAX_IDLE_TYPES];
unsigned int lb_imbalance[CPU_MAX_IDLE_TYPES];
unsigned int lb_gained[CPU_MAX_IDLE_TYPES];
unsigned int lb_hot_gained[CPU_MAX_IDLE_TYPES];
unsigned int lb_nobusyg[CPU_MAX_IDLE_TYPES];
unsigned int lb_nobusyq[CPU_MAX_IDLE_TYPES];
/* Active load balancing */
unsigned int alb_count;
unsigned int alb_failed;
unsigned int alb_pushed;
/* SD_BALANCE_EXEC stats */
unsigned int sbe_count;
unsigned int sbe_balanced;
unsigned int sbe_pushed;
/* SD_BALANCE_FORK stats */
unsigned int sbf_count;
unsigned int sbf_balanced;
unsigned int sbf_pushed;
/* try_to_wake_up() stats */
unsigned int ttwu_wake_remote;
unsigned int ttwu_move_affine;
unsigned int ttwu_move_balance;
#endif
[...snipped...]
};
3.2 Per task statistic
/proc/<pid>/schedstat
Common for all scheduling classes.
The statistics for /proc/<pid>/schedstat is defined as member of struct task_struct in include/linux/sched.h,
#if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
struct sched_info {
/* cumulative counters */
unsigned long pcount; /* # of times run on this cpu */
unsigned long long run_delay; /* time spent waiting on a runqueue */
/* timestamps */
unsigned long long last_arrival,/* when we last ran on a cpu */
last_queued; /* when we were last queued to run */
};
#endif /* defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT) */
struct task_struct {
[...snipped...]
#if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
struct sched_info sched_info;
#endif
[...snipped...]
};
/proc/<pid>/sched
Only available for CFS tasks. Need enable SCHED_DEBUG as well.
The se statistics for /proc/<pid>/sched is defined as member of struct task_struct in include/linux/sched.h,
#ifdef CONFIG_SCHEDSTATS
struct sched_statistics {
u64 wait_start;
u64 wait_max;
u64 wait_count;
u64 wait_sum;
u64 iowait_count;
u64 iowait_sum;
u64 sleep_start;
u64 sleep_max;
s64 sum_sleep_runtime;
u64 block_start;
u64 block_max;
u64 exec_max;
u64 slice_max;
u64 nr_migrations_cold;
u64 nr_failed_migrations_affine;
u64 nr_failed_migrations_running;
u64 nr_failed_migrations_hot;
u64 nr_forced_migrations;
u64 nr_wakeups;
u64 nr_wakeups_sync;
u64 nr_wakeups_migrate;
u64 nr_wakeups_local;
u64 nr_wakeups_remote;
u64 nr_wakeups_affine;
u64 nr_wakeups_affine_attempts;
u64 nr_wakeups_passive;
u64 nr_wakeups_idle;
};
#endif
struct sched_entity {
[...snipped...]
#ifdef CONFIG_SCHEDSTATS
struct sched_statistics statistics;
#endif
[...snipped...]
};
struct task_struct {
[...snipped...]
struct sched_entity se;
[...snipped...]
};
4. SCHEDSTATS source files
To use SCHEDSTATS, need to enable kernel config SCHEDSTATS. All related code is protected by CONFIG_SCHEDSTATS.
As far as we know, Linux kernel scheduler defined two layers,
4.1 The upper layer - scheduler core
This is a common layer for all scheduling class.
In Linux 3.2.x, The SCHEDSTATS source files in scheduler common layer are,
include/linux/sched.h
Per-sched-domain and per-task perf counters definitions.
kernel/sched_stats.h
/proc/schestat proc file implementation
fs/proc/base.c
/proc/<pid>/schedstat proc file implementation
kernel/sched.c
Per-runqueue perf counters definitions.
Per-runqueue, per-sched-domain, per-task perf counters implementation, for example, ttwu_stat
kernel/profile.c
The legacy code, profiling code for /proc/profile support, readprofile(1) could read it.
kernel/sched_debug.c
SCHEDSTATS in /proc/sched_debug and /proc/<pid>/sched proc files implementation. Need enable SCHED_DEBUG at same time.
4.2 The underlying layer - per scheduling class
In Linux 3.2.x, only the CFS scheduling class code has the SCHEDSTATS implementation.
kernel/sched_fair.c
SCHEDSTATS in /proc/<pid>/sched. Need enable SCHED_DEBUG at same time.
/proc/schedstat counters for load balance.
Kernel Trace points for wait, sleep, iowait, blocked(not in 3.2.x) events. See section 3 in this blog.