linux-kernel - Re: [RFC v3 1/6] Track the active utilisation

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Date:   Tue, 6 Dec 2016 09:35:01 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     luca abeni <luca.abeni@...tn.it>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@....com>,
        Claudio Scordino <claudio@...dence.eu.com>
Subject: Re: [RFC v3 1/6] Track the active utilisation

On Mon, Dec 05, 2016 at 11:30:05PM +0100, luca abeni wrote:
> Hi Peter,
> 
> On Fri, 18 Nov 2016 15:23:59 +0100
> Peter Zijlstra <peterz@...radead.org> wrote:
> [...]
> > 	u64 running_bw;
> > 
> > static void add_running_bw(struct sched_dl_entity *dl_se, struct
> > dl_rq *dl_rq) {
> > 	u64 old = dl_rq->running_bw;
> > 
> > 	dl_rq->running_bw += dl_se->dl_bw;
> > 	SCHED_WARN_ON(dl_rq->running_bw < old); /* overflow */
> > }
> > 
> > static void sub_running_bw(struct sched_dl_entity *dl_se, struct
> > dl_rq *dl_rq) {
> > 	u64 old = dl_rq->running_bw;
> > 
> > 	dl_rq->running_bw -= dl_se->dl_bw;
> > 	SCHED_WARN_ON(dl_rq->running_bw > old); /* underflow */
> > }
> 
> I wanted to change "SCHED_WARN_ON(dl_rq->running_bw > old); /*
> underflow */" into "if (SCHED_WARN_ON(...)) dl_rq->running_bw = 0" (to
> avoid using nonsensical "running_bw" values), but I see that
> "SCHED_WARN_ON()" cannot be used inside an if (this seems to be a
> difference respect to "SCHED_WARN()").

There's a SCHED_WARN? Did you mean to say WARN_ON()?

And yes, mostly by accident I think, I'm not a big user of that pattern
and neglected it when I did SCHED_WARN_ON().

> This is because of the definition used when CONFIG_SCHED_DEBUG is not
> defined (I noticed the issue when testing with random kernel
> configurations).

I'm fine changing the definition, just find something that works. The
current ((void)(x)) thing was to avoid unused complaints -- although I'm
not sure there were any.