linux-kernel - Re: [PATCH 1/2] sched/fair: Couple wakee flips with heavy wakers

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Date:   Fri, 29 Oct 2021 09:42:19 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Tao Zhou <tao.zhou@...ux.dev>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Mike Galbraith <efault@....de>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] sched/fair: Couple wakee flips with heavy wakers

On Fri, Oct 29, 2021 at 12:19:48AM +0800, Tao Zhou wrote:
> Hi Mel,
> 
> On Thu, Oct 28, 2021 at 10:48:33AM +0100, Mel Gorman wrote:
> 
> > @@ -5865,6 +5865,14 @@ static void record_wakee(struct task_struct *p)
> >  	}
> >  
> >  	if (current->last_wakee != p) {
> > +		int min = __this_cpu_read(sd_llc_size) << 1;
> > +		/*
> > +		 * Couple the wakee flips to the waker for the case where it
> > +		 * doesn't accrue flips, taking care to not push the wakee
> > +		 * high enough that the wake_wide() heuristic fails.
> > +		 */
> > +		if (current->wakee_flips > p->wakee_flips * min)
> > +			p->wakee_flips++;
> >  		current->last_wakee = p;
> >  		current->wakee_flips++;
> >  	}
> > @@ -5895,7 +5903,7 @@ static int wake_wide(struct task_struct *p)
> >  
> >  	if (master < slave)
> >  		swap(master, slave);
> > -	if (slave < factor || master < slave * factor)
> > +	if ((slave < factor && master < (factor>>1)*factor) || master < slave * factor)
> 
> So, the check like this include the above range:
> 
>   if ((slave < factor && master < slave * factor) ||
>        master < slave * factor)
> 
> That "factor>>1" filter some.
> 
> If "slave < factor" is true and "master < (factor>>1)*factor" is false,
> then we check "master < slave * factor".(This is one path added by the
> check "&&  master < (factor>>1)*factor").
> In the latter check "slave < factor" must be true, the result of this
> check depend on slave in the range [factor, factor>>1] if there is possibility
> that "master < slave * factor". If slave in [factor>>1, 0], the check of
> "master < slave * factor" is absolutly false and this can be filtered if
> we use a variable to load the result of master < (factor>>1)*factor.
> 
> My random random inputs and continue confusing to move on.
> 

I'm not sure what point you're trying to make.

-- 
Mel Gorman
SUSE Labs