linux-kernel - Re: [RFC PATCH v7] sched/fair: select idle cpu from idle cpumask for task wakeup

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Message-ID: <20201214093207.GY3040@hirez.programming.kicks-ass.net>
Date:   Mon, 14 Dec 2020 10:32:07 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     "Li, Aubrey" <aubrey.li@...ux.intel.com>, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        valentin.schneider@....com, qais.yousef@....com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        tim.c.chen@...ux.intel.com, linux-kernel@...r.kernel.org,
        Mel Gorman <mgorman@...e.de>, Jiang Biao <benbjiang@...il.com>
Subject: Re: [RFC PATCH v7] sched/fair: select idle cpu from idle cpumask for
 task wakeup

On Fri, Dec 11, 2020 at 10:50:02PM +0000, Mel Gorman wrote:

> > > The third potential downside is that the SMT sibling is not guaranteed to
> > > be checked due to SIS_PROP throttling but in the old code, that would have
> > > been checked by select_idle_smt(). That might result in premature stacking
> > > of runnable tasks on the same CPU. Similarly, as __select_idle_core may
> > > find multiple idle candidates, it will not pick the targets SMT sibling
> > > if it is idle like select_idle_smt would have.
> > > 
> > > That said, I am skeptical that select_idle_smt() matters all that often.
> > 
> > This, I didn't really believe in it either.
> > 
> 
> Good because I think any benefit from select_idle_smt is so marginal
> that it should be ignored if the full scan is simpler overall.

Perhaps we should start out with a simple patch removing that pass..
That should show, what, if anything, the effect of it is.