Documentation/scheduler/sched-stats.rst

====================
Scheduler Statistics
====================

Version 16 of schedstats changed the order of definitions within
'enum cpu_idle_type', which changed the order of [CPU_MAX_IDLE_TYPES]
columns in show_schedstat(). In particular the position of CPU_IDLE
and __CPU_NOT_IDLE changed places. The size of the array is unchanged.

Version 15 of schedstats dropped counters for some sched_yield:
yld_exp_empty, yld_act_empty and yld_both_empty. Otherwise, it is
identical to version 14.

Version 14 of schedstats includes support for sched_domains, which hit the
mainline kernel in 2.6.20 although it is identical to the stats from version
12 which was in the kernel from 2.6.13-2.6.19 (version 13 never saw a kernel
release).  Some counters make more sense to be per-runqueue; other to be
per-domain.  Note that domains (and their associated information) will only
be pertinent and available on machines utilizing CONFIG_SMP.

In version 14 of schedstat, there is at least one level of domain
statistics for each cpu listed, and there may well be more than one
domain.  Domains have no particular names in this implementation, but
the highest numbered one typically arbitrates balancing across all the
cpus on the machine, while domain0 is the most tightly focused domain,
sometimes balancing only between pairs of cpus.  At this time, there
are no architectures which need more than three domain levels. The first
field in the domain stats is a bit map indicating which cpus are affected
by that domain.

These fields are counters, and only increment.  Programs which make use
of these will need to start with a baseline observation and then calculate
the change in the counters at each subsequent observation.  A perl script
which does this for many of the fields is available at

    http://eaglet.pdxhosts.com/rick/linux/schedstat/

Note that any such script will necessarily be version-specific, as the main
reason to change versions is changes in the output format.  For those wishing
to write their own scripts, the fields are described here.

CPU statistics
--------------
cpu<N> 1 2 3 4 5 6 7 8 9

First field is a sched_yield() statistic:

     1) # of times sched_yield() was called

Next three are schedule() statistics:

     2) This field is a legacy array expiration count field used in the O(1)
	scheduler. We kept it for ABI compatibility, but it is always set to zero.
     3) # of times schedule() was called
     4) # of times schedule() left the processor idle

Next two are try_to_wake_up() statistics:

     5) # of times try_to_wake_up() was called
     6) # of times try_to_wake_up() was called to wake up the local cpu

Next three are statistics describing scheduling latency:

     7) sum of all time spent running by tasks on this processor (in nanoseconds)
     8) sum of all time spent waiting to run by tasks on this processor (in
        nanoseconds)
     9) # of timeslices run on this cpu


Domain statistics
-----------------
One of these is produced per domain for each cpu described. (Note that if
CONFIG_SMP is not defined, *no* domains are utilized and these lines
will not appear in the output.)

domain<N> <cpumask> 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

The first field is a bit mask indicating what cpus this domain operates over.

The next 24 are a variety of sched_balance_rq() statistics in grouped into types
of idleness (idle, busy, and newly idle):

    1)  # of times in this domain sched_balance_rq() was called when the
        cpu was idle
    2)  # of times in this domain sched_balance_rq() checked but found
        the load did not require balancing when the cpu was idle
    3)  # of times in this domain sched_balance_rq() tried to move one or
        more tasks and failed, when the cpu was idle
    4)  sum of imbalances discovered (if any) with each call to
        sched_balance_rq() in this domain when the cpu was idle
    5)  # of times in this domain pull_task() was called when the cpu
        was idle
    6)  # of times in this domain pull_task() was called even though
        the target task was cache-hot when idle
    7)  # of times in this domain sched_balance_rq() was called but did
        not find a busier queue while the cpu was idle
    8)  # of times in this domain a busier queue was found while the
        cpu was idle but no busier group was found
    9)  # of times in this domain sched_balance_rq() was called when the
        cpu was busy
    10) # of times in this domain sched_balance_rq() checked but found the
        load did not require balancing when busy
    11) # of times in this domain sched_balance_rq() tried to move one or
        more tasks and failed, when the cpu was busy
    12) sum of imbalances discovered (if any) with each call to
        sched_balance_rq() in this domain when the cpu was busy
    13) # of times in this domain pull_task() was called when busy
    14) # of times in this domain pull_task() was called even though the
        target task was cache-hot when busy
    15) # of times in this domain sched_balance_rq() was called but did not
        find a busier queue while the cpu was busy
    16) # of times in this domain a busier queue was found while the cpu
        was busy but no busier group was found

    17) # of times in this domain sched_balance_rq() was called when the
        cpu was just becoming idle
    18) # of times in this domain sched_balance_rq() checked but found the
        load did not require balancing when the cpu was just becoming idle
    19) # of times in this domain sched_balance_rq() tried to move one or more
        tasks and failed, when the cpu was just becoming idle
    20) sum of imbalances discovered (if any) with each call to
        sched_balance_rq() in this domain when the cpu was just becoming idle
    21) # of times in this domain pull_task() was called when newly idle
    22) # of times in this domain pull_task() was called even though the
        target task was cache-hot when just becoming idle
    23) # of times in this domain sched_balance_rq() was called but did not
        find a busier queue while the cpu was just becoming idle
    24) # of times in this domain a busier queue was found while the cpu
        was just becoming idle but no busier group was found

   Next three are active_load_balance() statistics:

    25) # of times active_load_balance() was called
    26) # of times active_load_balance() tried to move a task and failed
    27) # of times active_load_balance() successfully moved a task

   Next three are sched_balance_exec() statistics:

    28) sbe_cnt is not used
    29) sbe_balanced is not used
    30) sbe_pushed is not used

   Next three are sched_balance_fork() statistics:

    31) sbf_cnt is not used
    32) sbf_balanced is not used
    33) sbf_pushed is not used

   Next three are try_to_wake_up() statistics:

    34) # of times in this domain try_to_wake_up() awoke a task that
        last ran on a different cpu in this domain
    35) # of times in this domain try_to_wake_up() moved a task to the
        waking cpu because it was cache-cold on its own cpu anyway
    36) # of times in this domain try_to_wake_up() started passive balancing

/proc/<pid>/schedstat
---------------------
schedstats also adds a new /proc/<pid>/schedstat file to include some of
the same information on a per-process level.  There are three fields in
this file correlating for that process to:

     1) time spent on the cpu (in nanoseconds)
     2) time spent waiting on a runqueue (in nanoseconds)
     3) # of timeslices run on this cpu

A program could be easily written to make use of these extra fields to
report on how well a particular process or set of processes is faring
under the scheduler's policies.  A simple version of such a program is
available at

    http://eaglet.pdxhosts.com/rick/linux/schedstat/v12/latency.c