linux-kernel - Re: [PATCH] [RFC] sched: restrict iowait boost for boosted task only

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Message-ID: <20200124113050.i6ovkibcmutypm3q@e107158-lin>
Date:   Fri, 24 Jan 2020 11:30:51 +0000
From:   Qais Yousef <qais.yousef@....com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Quentin Perret <qperret@...gle.com>, Wei Wang <wvw@...gle.com>,
        wei.vince.wang@...il.com, dietmar.eggemann@....com,
        chris.redpath@....com, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [RFC] sched: restrict iowait boost for boosted task only

On 01/24/20 11:01, Valentin Schneider wrote:
> On 24/01/2020 09:51, Quentin Perret wrote:
> >>> +static inline bool iowait_boosted(struct task_struct *p)
> >>> +{
> >>> +	return p->in_iowait && uclamp_eff_value(p, UCLAMP_MIN) > 0;
> >>
> >> I think this is overloading the usage of util clamp. You're basically using
> >> cpu.uclamp.min to temporarily switch iowait boost on/off.
> >>
> >> Isn't it better to add a new cgroup attribute to toggle this feature?
> >>
> >> The problem does seem generic enough and could benefit other battery-powered
> >> devices outside of the Android world. I don't think the dependency on uclamp &&
> >> energy model are necessary to solve this.
> > 
> > I think using uclamp is not a bad idea here, but perhaps we could do
> > things differently. As of today the iowait boost escapes the clamping
> > mechanism, so one option would be to change that. That would let us set
> > a low max clamp in the 'background' cgroup, which in turns would limit
> > the frequency request for those tasks even if they're IO-intensive.
> > 
> 
> So I'm pretty sure we *do* want tasks with the default clamps to get iowait
> boost'd. What we don't want are background tasks driving up the frequency,
> and that should be via uclamp.max (as Quentin is suggesting) rather than
> uclamp.min (as is suggested in the patch).
> 
> Now, whether that is overloading the usage of uclamp... I'm not sure.
> One of the argument for uclamp was actually frequency selection, so if
> we just make iowait boost respect that, IOW not boost further than
> uclamp.max (which is a bit better than a simple on/off switch), that
> wouldn't be too crazy I think.

Capping iowait boost value in schedutil based on uclamp makes sense indeed.

What didn't make sense to me is the use of uclamp as a switch to toggle iowait
boost on/off.

Cheers

--
Qais Yousef