linux-kernel - Re: [PATCH] sched/rt: Add pid and comm when RT throttling activated

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Date:   Fri, 9 Dec 2022 16:36:06 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Yajun Deng <yajun.deng@...ux.dev>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        vschneid@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/rt: Add pid and comm when RT throttling activated

On Tue,  6 Dec 2022 16:36:41 +0800
Yajun Deng <yajun.deng@...ux.dev> wrote:

> It is difficult to know which process consumes time when RT throttling
> activated.
> 
> So add pid and comm for this case.
> 

Unfortunately, that only shows the process that "broke the camel's
back"[1]. The things is, it is the amount of time *all* RT tasks use to
trigger throttling. You are only printing the task that the throttling
occurred on.

Try running https://rostedt.org/code/migrate.c . It will create a bunch of
RT tasks, and they all will throttle once it hits 950ms ever second.

  trace-cmd record -e sched_switch ./migrate
  kernelshark

Then plot all the migrate tasks.

The above will demonstrate it very well.

-- Steve


[1] https://en.wikipedia.org/wiki/Straw_that_broke_the_camel%27s_back

> Signed-off-by: Yajun Deng <yajun.deng@...ux.dev>
> ---
>  kernel/sched/rt.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index ed2a47e4ddae..1f267ab1b59a 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1012,6 +1012,7 @@ static int sched_rt_runtime_exceeded(struct rt_rq *rt_rq)
>  
>  	if (rt_rq->rt_time > runtime) {
>  		struct rt_bandwidth *rt_b = sched_rt_bandwidth(rt_rq);
> +		struct task_struct *curr = rq_of_rt_rq(rt_rq)->curr;
>  
>  		/*
>  		 * Don't actually throttle groups that have no runtime assigned
> @@ -1019,7 +1020,8 @@ static int sched_rt_runtime_exceeded(struct rt_rq *rt_rq)
>  		 */
>  		if (likely(rt_b->rt_runtime)) {
>  			rt_rq->rt_throttled = 1;
> -			printk_deferred_once("sched: RT throttling activated\n");
> +			printk_deferred_once("pid: %d, comm: %s, sched: RT throttling activated\n",
> +					     curr->pid, curr->comm);
>  		} else {
>  			/*
>  			 * In case we did anyway, make it go away,