Re: [PATCH 3/6] sched: make struct sched_statistics independent of fair sched class

[Mel Gorman]

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 <laoar.shao@xxxxxxxxx>

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@xxxxxxx>

-- 
Mel Gorman
SUSE Labs