linux-kernel - Re: [RFC PATCH V2] sched: psi: Add psi events trace point

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Message-ID: <CAJuCfpE1pJ8qhHgqvExktsMeTBbtVSK2rkE5SfeTE2nOYrNozQ@mail.gmail.com>
Date: Wed, 20 Aug 2025 12:51:37 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Xuewen Yan <xuewen.yan@...soc.com>
Cc: hannes@...xchg.org, peterz@...radead.org, mathieu.desnoyers@...icios.com, 
	mhiramat@...nel.org, rostedt@...dmis.org, mingo@...hat.com, 
	juri.lelli@...hat.com, vincent.guittot@...aro.org, dietmar.eggemann@....com, 
	bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com, 
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
	ke.wang@...soc.com, yuming.han@...soc.com, xuewen.yan94@...il.com
Subject: Re: [RFC PATCH V2] sched: psi: Add psi events trace point

On Wed, Aug 20, 2025 at 4:28 AM Xuewen Yan <xuewen.yan@...soc.com> wrote:
>
> Add trace point to psi triggers. This is useful to
> observe the psi events in the kernel space.
>
> One use of this is to monitor memory pressure.
> When the pressure is too high, we can kill the process
> in the kernel space to prevent OOM.
>
> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
> ---
> v2:
> -fix compilation error;
> -export the tp;
> -add more commit message;
> ---
>  include/trace/events/sched.h | 5 +++++
>  kernel/sched/psi.c           | 4 ++++
>  2 files changed, 9 insertions(+)
>
> diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
> index 7b2645b50e78..d54db5fcbca2 100644
> --- a/include/trace/events/sched.h
> +++ b/include/trace/events/sched.h
> @@ -896,6 +896,11 @@ DECLARE_TRACE(sched_set_need_resched,
>         TP_PROTO(struct task_struct *tsk, int cpu, int tif),
>         TP_ARGS(tsk, cpu, tif));
>
> +struct psi_trigger;
> +DECLARE_TRACE(psi_event,

DECLARE_TRACE will create a tracepoint but will not export it in the
tracefs. Why should we not have it in the tracefs?

> +       TP_PROTO(struct psi_trigger *t),
> +       TP_ARGS(t));
> +
>  #endif /* _TRACE_SCHED_H */
>
>  /* This part must be outside protection */
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index 59fdb7ebbf22..f06eb91a1250 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -141,6 +141,8 @@
>  #include <linux/psi.h>
>  #include "sched.h"
>
> +EXPORT_TRACEPOINT_SYMBOL_GPL(psi_event_tp);

So, are you planning to attach some handler to this trace event in your driver?

> +
>  static int psi_bug __read_mostly;
>
>  DEFINE_STATIC_KEY_FALSE(psi_disabled);
> @@ -509,6 +511,8 @@ static void update_triggers(struct psi_group *group, u64 now,
>                 if (now < t->last_event_time + t->win.size)
>                         continue;
>
> +               trace_psi_event_tp(t);

This should only be done if the below cmpxchg() check is true, right?
Otherwise it will not match with what userspace is receiving.

> +
>                 /* Generate an event */
>                 if (cmpxchg(&t->event, 0, 1) == 0) {
>                         if (t->of)
> --
> 2.25.1
>