linux-kernel - Re: [PATCH v4 06/24] sched/fair: Collect load-balancing stats for IPC classes

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Date:   Thu, 6 Jul 2023 16:40:59 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Ionela Voinescu <ionela.voinescu@....com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ricardo Neri <ricardo.neri@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Len Brown <len.brown@...el.com>, Mel Gorman <mgorman@...e.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Valentin Schneider <vschneid@...hat.com>,
        Lukasz Luba <lukasz.luba@....com>,
        Zhao Liu <zhao1.liu@...el.com>,
        "Yuan, Perry" <Perry.Yuan@....com>, x86@...nel.org,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        "Tim C . Chen" <tim.c.chen@...el.com>,
        Zhao Liu <zhao1.liu@...ux.intel.com>
Subject: Re: [PATCH v4 06/24] sched/fair: Collect load-balancing stats for
 IPC classes

On Mon, Jun 26, 2023 at 12:52:59PM -0700, Tim Chen wrote:
> 
> > > 
> > > nit: the comment is unnecessary, and a bit misleading, IMO.
> > > 
> > > The comment says "This group will not be selected." but the only way to
> > > guarantee that here is to reset the sum_score to 0 when you find an
> > > invalid score, which I don't believe is your intention.
> > 
> > You raise an interesting point. We will call this function for each rq
> > in the sched group. Thus, if we encounter an error, the scores would be
> > incomplete. Therefore, I think that the scores should be discarded and
> > reset to 0 so that they are not used, IMO.
> 
> Since we still have other rqs to loop through, reset to 0 here does
> not guarantee that it will stay that way at the end of the loop when
> the sum could still be added to.

That is true, to discard the would need an indication that we should not
keep iterating on the runqueues of this group.

> May need a special value like -EINVAL
> and make the score a "long" to mark such case.

IIUC, unsigned longs allow to handle negative errors if you use
IS_ERR_VALUE(); but yes, it looks odd.