linux-kernel - Re: [PATCH] sched/fair: Disable LB

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Message-ID: <20180911113058.GQ24106@hirez.programming.kicks-ass.net>
Date:   Tue, 11 Sep 2018 13:30:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Li Zhijian <zhijianx.li@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Disable LB_BIAS by default

On Tue, Sep 11, 2018 at 08:22:49AM +0200, Ingo Molnar wrote:
> 
> * Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Thu, Aug 09, 2018 at 02:57:53PM +0100, Dietmar Eggemann wrote:
> > > LB_BIAS allows the adjustment on how conservative load should be
> > > balanced.
> > 
> > > It is very likely that LB_BIAS' influence on load balancing can be
> > > neglected (see test results below). This is further supported by:
> > > 
> > > (1) Weighted CPU load today is by itself a decayed average value (PELT)
> > >     (cfs_rq->avg->runnable_load_avg) and not the instantaneous load
> > >     (rq->load.weight) it was when LB_BIAS was introduced.
> > > 
> > > (2) Sd imbalance_pct is used for CPU_NEWLY_IDLE and CPU_NOT_IDLE (relate
> > >     to sd's newidle and busy idx) in find_busiest_group() when comparing
> > >     busiest and local avg load to make load balancing even more
> > >     conservative.
> > > 
> > > (3) The sd forkexec and newidle idx are always set to 0 so there is no
> > >     adjustment on how conservatively load balancing is done here.
> > > 
> > > (4) Affine wakeup based on weight (wake_affine_weight()) will not be
> > >     impacted since the sd wake idx is always set to 0.
> > > 
> > > Let's disable LB_BIAS by default for a few kernel releases to make sure
> > > that no workload and no scheduler topology is affected. The benefit of
> > > being able to remove the LB_BIAS dependency from source_load() and
> > > target_load() is that the entire rq->cpu_load[idx] code could be removed
> > > in this case.
> > 
> > Certainly worth a try; as I've written somewhere in a comment; it would
> > be very nice to get rid of that load tracking crud.
> > 
> > And it is trivial to revert if something does show up.
> > 
> > Ingo, what do you think?
> 
> Ack, I'm very much in favor of reducing complexity.

OK, I've queued it up.