linux-kernel - Re: [PATCH 4/7] sched: Add sched_load

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Date:   Mon, 6 May 2019 15:38:12 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org,
        Pavankumar Kondeti <pkondeti@...eaurora.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Uwe Kleine-Konig <u.kleine-koenig@...gutronix.de>
Subject: Re: [PATCH 4/7] sched: Add sched_load_rq tracepoint

On 05/06/19 11:08, Peter Zijlstra wrote:
> On Sun, May 05, 2019 at 12:57:29PM +0100, Qais Yousef wrote:
> 
> > +/*
> > + * Following tracepoints are not exported in tracefs and provide hooking
> > + * mechanisms only for testing and debugging purposes.
> > + */
> > +DECLARE_TRACE(sched_load_rq,
> > +	TP_PROTO(int cpu, const char *path, struct sched_avg *avg),
> > +	TP_ARGS(cpu, path, avg));
> > +
> 
> > +DECLARE_TRACE(sched_load_se,
> > +       TP_PROTO(int cpu, const char *path, struct sched_entity *se),
> > +       TP_ARGS(cpu, path, se));
> > +
> 
> > +DECLARE_TRACE(sched_overutilized,
> > +       TP_PROTO(int overutilized),
> > +       TP_ARGS(overutilized));
> 
> This doesn't generate any actual userspace because of the lack of
> DEFINE_EVENT() ?

Documentation/trace/tracepoints.rst suggests using DEFINE_TRACE(). But using
that causes compilation errors because of some magic that is being redefined.
Not doing DEFINE_TRACE() gave the intended effect according to the document, so
I assumed it's outdated.


kernel/sched/core.c:27:1: note: in expansion of macro ‘DEFINE_TRACE’
 DEFINE_TRACE(sched_overutilized);
 ^~~~~~~~~~~~
./include/linux/tracepoint.h:287:20: note: previous definition of ‘__tracepoint_sched_overutilized’ was here
  struct tracepoint __tracepoint_##name     \
                    ^
./include/linux/tracepoint.h:293:2: note: in expansion of macro ‘DEFINE_TRACE_FN’
  DEFINE_TRACE_FN(name, NULL, NULL);
  ^~~~~~~~~~~~~~~
./include/trace/define_trace.h:67:2: note: in expansion of macro ‘DEFINE_TRACE’
  DEFINE_TRACE(name)
  ^~~~~~~~~~~~
./include/trace/events/sched.h:603:1: note: in expansion of macro ‘DECLARE_TRACE’
 DECLARE_TRACE(sched_overutilized,
 ^~~~~~~~~~~~~


DEFINE_EVENT() is only used with TRACE_EVENT() so certainly we don't want it
here.

> 
> > diff --git a/kernel/sched/sched_tracepoints.h b/kernel/sched/sched_tracepoints.h
> > new file mode 100644
> > index 000000000000..f4ded705118e
> > --- /dev/null
> > +++ b/kernel/sched/sched_tracepoints.h
> > @@ -0,0 +1,39 @@
> 
> Like with the other newly introduced header files, this one is lacking
> the normal include guard.

I was going to add them but then when I looked in sched.h and autogroup.h they
had none. So I thought the convention is to not use guard here.

I will add it.

> 
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +/*
> > + * Scheduler tracepoints that are probe-able only and aren't exported ABI in
> > + * tracefs.
> > + */
> > +
> > +#include <trace/events/sched.h>
> > +
> > +#define SCHED_TP_PATH_LEN		64
> > +
> > +
> > +static __always_inline void sched_tp_load_cfs_rq(struct cfs_rq *cfs_rq)
> > +{
> > +	if (trace_sched_load_rq_enabled()) {
> > +		int cpu = cpu_of(rq_of(cfs_rq));
> > +		char path[SCHED_TP_PATH_LEN];
> > +
> > +		cfs_rq_tg_path(cfs_rq, path, SCHED_TP_PATH_LEN);
> > +		trace_sched_load_rq(cpu, path, &cfs_rq->avg);
> > +	}
> > +}
> > +
> > +static __always_inline void sched_tp_load_rt_rq(struct rq *rq)
> > +{
> > +	if (trace_sched_load_rq_enabled()) {
> > +		int cpu = cpu_of(rq);
> > +
> > +		trace_sched_load_rq(cpu, NULL, &rq->avg_rt);
> > +	}
> > +}
> > +
> > +static __always_inline void sched_tp_load_dl_rq(struct rq *rq)
> > +{
> > +	if (trace_sched_load_rq_enabled()) {
> > +		int cpu = cpu_of(rq);
> > +
> > +		trace_sched_load_rq(cpu, NULL, &rq->avg_dl);
> > +	}
> > +}
> 
> > +static __always_inline void sched_tp_load_se(struct sched_entity *se)
> > +{
> > +       if (trace_sched_load_se_enabled()) {
> > +               struct cfs_rq *gcfs_rq = group_cfs_rq(se);
> > +               struct cfs_rq *cfs_rq = cfs_rq_of(se);
> > +               char path[SCHED_TP_PATH_LEN];
> > +               int cpu = cpu_of(rq_of(cfs_rq));
> > +
> > +               cfs_rq_tg_path(gcfs_rq, path, SCHED_TP_PATH_LEN);
> > +               trace_sched_load_se(cpu, path, se);
> > +       }
> > +}
> 
> These functions really should be called trace_*()

I can rename the wrappers to trace_pelt_load_rq() or sched_trace_pelt_load_rq()
as Steve was suggesting.

I assume you're okay with the name of the tracepoints and your comment was
about the wrapper above only? ie: sched_load_rq vs pelt_rq.

> 
> Also; I _really_ hate how fat they are. Why can't we do simple straight

We can create a percpu variable instead of pushing the path on the stack. But
this might fail if the trancepoint is called in a preempt enabled path. Also
having the percpu variable always hanging when these mostly disabled is ugly.

Maybe there's a better way to handle extracting this path info without copying
it here. Let me see if I can improve on this.

> forward things like:
> 
> 	trace_pelt_cfq(cfq);
> 	trace_pelt_rq(rq);
> 	trace_pelt_se(se);
> 
> And then have the thing attached to the event do the fat bits like
> extract the path and whatnot.

Thanks

--
Qais Yousef