linux-kernel - Re: [RFC PATCH 2/5] sched/events: Introduce cfs

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Date:   Tue, 28 Mar 2017 13:36:26 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@....com>,
        Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [RFC PATCH 2/5] sched/events: Introduce cfs_rq load tracking
 trace event

On Tue, 28 Mar 2017 18:44:59 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, Mar 28, 2017 at 10:46:00AM -0400, Steven Rostedt wrote:
> > On Tue, 28 Mar 2017 07:35:38 +0100
> > Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> >   
> > >  /* This part must be outside protection */
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 03adf9fb48b1..ac19ab6ced8f 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -2950,6 +2950,9 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa,
> > >  		sa->util_avg = sa->util_sum / LOAD_AVG_MAX;
> > >  	}
> > >  
> > > +	if (cfs_rq)
> > > +		trace_sched_load_cfs_rq(cfs_rq);
> > > +  
> > 
> > Please use TRACE_EVENT_CONDITION(), and test for cfs_rq not NULL.  
> 
> I too suggested that; but then I looked again at that code and we can
> actually do this. cfs_rq can be constant propagated and the if
> determined at build time.
> 
> Its not immediately obvious from the current code; but if we do
> something like the below, it should be clearer.
> 

But why play games, and rely on the design of the code? A
TRACE_EVENT_CONDTION() is more robust and documents that this
tracepoint should not be called when cfs_rq is NULL.

-- Steve