linux-kernel - Re: 答复: [PATCH V4] sched/cpufreq: initialize iowait_boost_max and iowait

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Message-ID: <20190304184708.GQ32494@hirez.programming.kicks-ass.net>
Date:   Mon, 4 Mar 2019 19:47:08 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Quentin Perret <quentin.perret@....com>
Cc:     Wang, Vincent (王争) 
        <Vincent.Wang@...soc.com>,
        Zhang, Chunyan (张春艳) 
        <Chunyan.Zhang@...soc.com>, Ingo Molnar <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Chunyan Zhang <zhang.lyra@...il.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: 答复: [PATCH V4] sched/cpufreq: initialize iowait_boost_max and
 iowait_boost with cpu capacity

On Mon, Mar 04, 2019 at 05:50:32PM +0000, Quentin Perret wrote:
> On Monday 04 Mar 2019 at 18:40:28 (+0100), Peter Zijlstra wrote:
> > > Perhaps you could keep the 'util' and 'max' pointers in
> > > sugov_iowait_apply() and overwrite them like before, but in the
> > > SCHED_CAPACITY_SCALE scale as you suggest ?
> > 
> > Urgh; but then we're back to having that boostrap problem.
> 
> Hmm, I don't understand :/

Yeah, I seen to have reading comprehension issues today. Ignore that.

> > Now; at this time; @max is in fact scale_cpu_capacity, so can't we
> > change this:
> > 
> > -       /*
> > -        * Apply the current boost value: a CPU is boosted only if its current
> > -        * utilization is smaller then the current IO boost level.
> > -        */
> > -       boost_util = sg_cpu->iowait_boost;
> > -       boost_max = sg_cpu->iowait_boost_max;
> 
> I was basically suggesting to do 'boost_max = 1024;' here and you
> should be good with you way of computing 'min' no ?

Right, but then we keep having to retain those two mults.

> > -       if (*util * boost_max < *max * boost_util) {
> > -               *util = boost_util;
> > -               *max = boost_max;
> > -       }
> > +       sg_cpu->iowait_boost_pending = false;
> > +
> > +       return min(max(util, sg_cpu->iowait_boost), max);
> > }
> > 
> > to something like:
> > 
> > 	/*
> > 	 * @util is already in capacity scale, convert iowait_boost
> > 	 * into the same scale so we can compare.
> > 	 */
> > 	boost = (sg_cpu->iowait_boost * max) >> SCHED_CAPACITY_SHIFT;
> > 	util = max(boost, util);
> > 	return min(util, max);
> > 
> 
> But this should work too, I think.

While that is only a single mult.