linux-kernel - Re: [PATCH v3 0/7] sched: support schedstats for RT sched class

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Message-ID: <YS3/jhuRNS2GGVrp@hirez.programming.kicks-ass.net>
Date:   Tue, 31 Aug 2021 12:08:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     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,
        achaiken@...ora.tech, lkp@...el.com, linux-kernel@...r.kernel.org,
        linux-rt-users@...r.kernel.org
Subject: Re: [PATCH v3 0/7] sched: support schedstats for RT sched class

On Tue, Aug 24, 2021 at 11:29:39AM +0000, Yafang Shao wrote:
> 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.

Do you really want schedstats or do you want the tracepoints? In general
I really want to cut back on the built-in statistics crud we carry,
there's too much and it seems to keep growing forever :-(

(as is the case here, you're extending it as well)

That said; making schedstats cover the other classes can be seen as
fixing an inconsistency, but then you forgot deadline.

> 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
>     ...
> };

Yeah, this seems to give a terrible mess, let me see if I can come up
with anything less horrible.