linux-kernel - Re: [RFC PATCH V3] sched: psi: Add psi events trace point

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Message-ID: <CAB8ipk-cCLWUg8A6tSMZW43fHj10XAzBU2O4Ed5TbDk73J8QBw@mail.gmail.com>
Date: Tue, 23 Sep 2025 14:07:32 +0800
From: Xuewen Yan <xuewen.yan94@...il.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Xuewen Yan <xuewen.yan@...soc.com>, hannes@...xchg.org, 
	mathieu.desnoyers@...icios.com, mhiramat@...nel.org, peterz@...radead.org, 
	rostedt@...dmis.org, mingo@...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
Subject: Re: [RFC PATCH V3] sched: psi: Add psi events trace point

On Tue, Sep 23, 2025 at 11:31 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Mon, Sep 22, 2025 at 7:58 PM Xuewen Yan <xuewen.yan94@...il.com> wrote:
> >
> > A very gentle ping on this patch.
> > Best regards,
> > Thanks!
>
> Thanks! Sorry I missed it.
Not at all, thanks for your reply:)

>
> >
> > On Thu, Sep 11, 2025 at 11:30 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>
> > > ---
> > > V3:
> > > -export it in the tracefs;
> > > ---
> > > v2:
> > > -fix compilation error;
> > > -export the tp;
> > > -add more commit message;
> > > ---
> > >  include/trace/events/sched.h | 27 +++++++++++++++++++++++++++
> > >  kernel/sched/psi.c           |  4 ++++
> > >  2 files changed, 31 insertions(+)
> > >
> > > diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
> > > index 7b2645b50e78..db8b8f25466e 100644
> > > --- a/include/trace/events/sched.h
> > > +++ b/include/trace/events/sched.h
> > > @@ -826,6 +826,33 @@ TRACE_EVENT(sched_wake_idle_without_ipi,
> > >         TP_printk("cpu=%d", __entry->cpu)
> > >  );
> > >
> > > +#ifdef CONFIG_PSI
> > > +TRACE_EVENT(psi_event,
> > > +
> > > +       TP_PROTO(int aggregator, int state, u64 threshold, u64 win_size),
> > > +
> > > +       TP_ARGS(aggregator, state, threshold, win_size),
> > > +
> > > +       TP_STRUCT__entry(
> > > +               __field(int,    aggregator)
> > > +               __field(int,    state)
> > > +               __field(u64,    threshold)
> > > +               __field(u64,    win_size)
> > > +       ),
> > > +
> > > +       TP_fast_assign(
> > > +               __entry->aggregator     = aggregator;
> > > +               __entry->state          = state;
> > > +               __entry->threshold      = threshold;
> > > +               __entry->win_size       = win_size;
> > > +       ),
> > > +
> > > +       TP_printk("aggregator=%d state=%d threshold=%llu window_size=%llu",
> > > +               __entry->aggregator, __entry->state, __entry->threshold,
> > > +               __entry->win_size)
> > > +);
> > > +#endif /* CONFIG_PSI */
> > > +
> > >  /*
> > >   * Following tracepoints are not exported in tracefs and provide hooking
> > >   * mechanisms only for testing and debugging purposes.
> > > diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> > > index 59fdb7ebbf22..5b7954b653ed 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);
> > > +
> > >  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(aggregator, t->state, t->threshold, t->win.size);
>
> Again, this trace event should be generated only after cmpxchg()
> passes the check. Otherwise trace events might be generated when
> actual PSI events are not (false positives). That disconnect is not
> acceptable.

You’re absolutely right. I’ll resolve it in the next patch.

Thanks!

BR
---
xuewen

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