Date:   Sun,  5 Sep 2021 14:35:39 +0000
From:   Yafang Shao <laoar.shao@...il.com>
To:     peterz@...radead.org, mingo@...hat.com, mgorman@...e.de,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        bristot@...hat.com
Cc:     linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
        achaiken@...ora.tech, Yafang Shao <laoar.shao@...il.com>
Subject: [PATCH v4 0/8] sched: support schedstats for RT sched class 

Why do we need schedstats ?
==========================

schedstats is a useful feature to do thread-level latency analysis. Our
usecase as follows,

  Userspace Code Scope         Profiler

  {
      user_func_abc(); <----   uprobe_scope_begin() get start schedstats 
      ...
      user_func_xyz(); <----   uprobe_scope_end() get end schedstats
  }

Then with the result of (end - begin) we can get below latency details
in a specific user scope,

  scope_latency = Wait + Sleep + Blocked [1]  + Run (stime + utime)

If there's no schedstats we have to trace the heavy sched::sched_switch
and do a lot more stuff. 

[1]. With patch #4 and don't include sum_block_runtime in sum_sleep_runtime

Support schedstats for RT sched class
=====================================

If we want to use the schedstats facility to trace 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. So we can
move it into struct task_struct and struct task_group to achieve the goal.

After the patch, schestats are orgnized as follows,

    struct task_struct {
       ...
       struct sched_entity se;
       struct sched_rt_entity rt;
       struct sched_dl_entity dl;
       ...
       struct sched_statistics stats;
       ...
   };

Regarding the task group, schedstats is only supported for fair group
sched, and a new struct sched_entity_stats is introduced, suggested by
Peter -

    struct sched_entity_stats {
        struct sched_entity     se;
        struct sched_statistics stats;
    } __no_randomize_layout;

Then with the se in a task_group, we can easily get the stats.

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.

As this patch changes the core struct of scheduler, so I verified the
performance it may impact on the scheduler with 'perf bench sched
pipe', suggested by Mel. Below is the result, in which all the values
are in usecs/op.
                                  Before               After
      kernel.sched_schedstats=0  5.2~5.4               5.2~5.4
      kernel.sched_schedstats=1  5.3~5.5               5.3~5.5
[These data is a little difference with the earlier version, that is
 because my old test machine is destroyed so I have to use a new
 different test machine.]

Almost no impact on the sched performance.

The user can get the schedstats information in the same way in fair sched
class. For example,
       fair                            RT
       /proc/[pid]/sched               /proc/[pid]/sched

schedstats is not supported for RT group.

The sched:sched_stat_{wait, sleep, iowait, blocked} tracepoints can
be used to trace RT tasks as well.

Support schedstats for any other sched classes
==============================================

After this patchset, it is very easy to extend the schedstats to any
other sched classes. The deadline sched class is also supported in this
patchset.

Changes Since v3:
Various code improvement per Peter,
- don't support schedstats for rt group
- introduce struct sched_entity_stats for fair group 
- change the position of 'struct sched_statistics stats'
- fixes indent issue
- change the output format in /proc/[pid]/sched
- add the usecase of schedstats
- support schedstats for deadline task
- and other suggestions

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 (8):
  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: 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
  sched, dl: support sched_stat_runtime tracepoint for deadline sched
    class
  sched, dl: support schedstats for deadline sched class

 include/linux/sched.h    |   8 +-
 kernel/sched/core.c      |  25 +++---
 kernel/sched/deadline.c  |  99 +++++++++++++++++++++-
 kernel/sched/debug.c     |  97 +++++++++++----------
 kernel/sched/fair.c      | 177 +++++++++++----------------------------
 kernel/sched/rt.c        | 130 +++++++++++++++++++++++++++-
 kernel/sched/stats.c     | 104 +++++++++++++++++++++++
 kernel/sched/stats.h     |  49 +++++++++++
 kernel/sched/stop_task.c |   4 +-
 9 files changed, 500 insertions(+), 193 deletions(-)

-- 
2.18.2

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