[PATCH v3 0/7] sched: support schedstats for RT sched class

[Yafang Shao]

Hi Ingo, Peter,

This feature is useful to trace the sched details of RT tasks. Hopefully
you can give some feedback on it.

We want to measure the latency of RT tasks in our production
environment with schedstats facility, but currently schedstats is only
supported for fair sched class. In order to support if for other sched
classes, we should make it independent of fair sched class. The struct
sched_statistics is the schedular statistics of a task_struct or a
task_group, both of which are independent of sched class. So we can move
struct sched_statistics into struct task_struct and struct task_group to
achieve the goal.

After the patchset, schestats are orgnized as follows,
struct task_struct {
    ...
    struct sched_statistics statistics;
    ...
    struct sched_entity *se;
    struct sched_rt_entity *rt;
    ...
};

struct task_group {                    |---> stats[0] : of CPU0
    ...                                |
    struct sched_statistics **stats; --|---> stats[1] : of CPU1
    ...                                |
                                       |---> stats[n] : of CPUn
 #ifdef CONFIG_FAIR_GROUP_SCHED
    struct sched_entity **se;
 #endif
 #ifdef CONFIG_RT_GROUP_SCHED
    struct sched_rt_entity  **rt_se;
 #endif
    ...
};

The sched_statistics members may be frequently modified when schedstats is
enabled, in order to avoid impacting on random data which may in the same
cacheline with them, the struct sched_statistics is defined as cacheline
aligned.

Then we can use schedstats to trace RT tasks as well, for example,
                    Interface File
 task schedstats :  /proc/[pid]/sched
 group schedstats:  /proc/sched_debug
 tracepoints     :  sched:sched_stat_{runtime, wait, sleep, iowait, blocked}

As PATCH #2 and #3 changes the core struct in the scheduler, so I did
'perf bench sched pipe' to measure the sched performance before and after
the change, suggested by Mel. Below is the data, which are all in
usecs/op.

                                  Before               After
      kernel.sched_schedstats=0  ~5.6                 ~5.6
      kernel.sched_schedstats=1  ~5.7                 ~5.7
    [These data is a little difference with the prev version, that is
     because my old test machine is destroyed so I have to use a new
     different test machine.]
No obvious difference after the change.

Changes Since v2:
- Fixes the output format in /proc/[pid]/sched  
- Rebase it on the latest code
- Redo the performance test

Changes since v1:
- Fix the build failure reported by kernel test robot.
- Add the performance data with 'perf bench sched pipe', suggested by
  Mel.
- Make the struct sched_statistics cacheline aligned.
- Introduce task block time in schedstats

Changes since RFC:
- improvement of schedstats helpers, per Mel.
- make struct schedstats independent of fair sched class 


Yafang Shao (7):
  sched, fair: use __schedstat_set() in set_next_entity()
  sched: make struct sched_statistics independent of fair sched class
  sched: make schedstats helpers independent of fair sched class
  sched: make the output of schedstats independent of fair sched class
  sched: introduce task block time in schedstats
  sched, rt: support sched_stat_runtime tracepoint for RT sched class
  sched, rt: support schedstats for RT sched class

 include/linux/sched.h    |   7 +-
 kernel/sched/core.c      |  24 +++--
 kernel/sched/deadline.c  |   4 +-
 kernel/sched/debug.c     | 136 +++++++++++++++----------
 kernel/sched/fair.c      | 210 ++++++++++++++++-----------------------
 kernel/sched/rt.c        | 147 ++++++++++++++++++++++++++-
 kernel/sched/sched.h     |   3 +
 kernel/sched/stats.c     | 104 +++++++++++++++++++
 kernel/sched/stats.h     |  89 +++++++++++++++++
 kernel/sched/stop_task.c |   4 +-
 10 files changed, 531 insertions(+), 197 deletions(-)

-- 
2.18.2