On Tue, Dec 01, 2020 at 07:54:13PM +0800, Yafang Shao wrote:

If we want to use schedstats facility, we should move out of
struct sched_statistics from the struct sched_entity or add it into other
sctructs of sched entity as well. Obviously the latter one is bad because
that requires more spaces. So we should move it into a common struct which
can be used by all sched classes.

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.

Below is the detailed explaination of the change in the structs.

- Before this patch

struct task_struct {            |-> struct sched_entity {
    ...                         |       ...
    struct sched_entity *se; ---|       struct sched_statistics statistics;
    struct sched_rt_entity *rt;         ...
    ...                                 ...
};                                  };

struct task_group {             |--> se[0]->statistics : schedstats of CPU0
    ...                         |
 #ifdef CONFIG_FAIR_GROUP_SCHED |
    struct sched_entity **se; --|--> se[1]->statistics : schedstats of CPU1
                                |
 #endif                         |
                                |--> se[N]->statistics : schedstats of CPUn

 #ifdef CONFIG_FAIR_GROUP_SCHED
    struct sched_rt_entity  **rt_se; (N/A)
 #endif
    ...
};

The '**se' in task_group is allocated in the fair sched class, which is
hard to be reused by other sched class.

- After this patch

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
    ...
};

After the patch it is clearly that both of se or rt_se can easily get the
sched_statistics by a task_struct or a task_group.

Signed-off-by: Yafang Shao <redacted>

I didn't see anything wrong as such, it's mostly a mechanical
conversion. The one slight caveat is the potential change in cache
location for the statistics but it's not necessarily negative. The stats
potentially move to a different cache line but it's less obvious whether
that even matters given the location is very similar.

There is increased overhead now when schedstats are *enabled* because
_schedstat_from_sched_entity() has to be called but it appears that it is
protected by a schedstat_enabled() check. So ok, schedstats when enabled
are now a bit more expensive but they were expensive in the first place
so does it matter?

I'd have been happied if there was a comparison with schedstats enabled
just in case the overhead is too high but it could also do with a second
set of eyeballs.

It's somewhat tentative but

Acked-by: Mel Gorman <mgorman@suse.de>

-- 
Mel Gorman
SUSE Labs