linux-kernel - Re: [PATCH v2 5/6] sched/rt: Better manage pushing unfit tasks on wakeup

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Date:   Wed, 26 Feb 2020 16:02:48 +0000
From:   Qais Yousef <qais.yousef@....com>
To:     Pavan Kondeti <pkondeti@...eaurora.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 5/6] sched/rt: Better manage pushing unfit tasks on
 wakeup

On 02/25/20 09:25, Pavan Kondeti wrote:
> > I haven't been staring at this code for as long as you, but since we have
> > logic at wakeup to do a push, I think we need something here anyway for unfit
> > tasks.
> > 
> > Fixing select_task_rq_rt() to better balance tasks will help a lot in general,
> > but if that was enough already then why do we need to consider a push at the
> > wakeup at all then?
> > 
> > AFAIU, in SMP the whole push-pull mechanism is racy and we introduce redundancy
> > at taking the decision on various points to ensure we minimize this racy nature
> > of SMP systems. Anything could have happened between the time we called
> > select_task_rq_rt() and the wakeup, so we double check again before we finally
> > go and run. That's how I interpret it.
> > 
> > I am open to hear about other alternatives first anyway. Your help has been
> > much appreciated so far.
> > 
> 
> The search inside find_lowest_rq() happens without any locks so I believe it
> is expected to have races like this. In fact there is a comment in the code
> saying "This test is optimistic, if we get it wrong the load-balancer
> will have to sort it out" in select_task_rq_rt(). However, the push logic
> as of today works only for overloaded case. In that sense, your patch fixes
> this race for b.L systems. At the same time, I feel like tracking nonfit tasks
> just to fix this race seems to be too much. I will leave this to Steve and
> others to take a decision.

I do think without this tasks can end up on the wrong CPU longer than they
should. Keep in mind that if a task is boosted to run on a big core, it still
have to compete with non-boosted tasks who can run on a any cpu. So this
opportunistic push might be necessary.

For 5.6 though, I'll send an updated series that removes the fitness check from
task_woken_rt() && switched_to_rt() and carry on with this discussion for 5.7.

> 
> I thought of suggesting to remove the below check from select_task_rq_rt()
> 
> p->prio < cpu_rq(target)->rt.highest_prio.curr
> 
> which would then make the target CPU overloaded and the push logic would
> spread the tasks. That works for a b.L system too. However there seems to
> be a very good reason for doing this. see
> https://lore.kernel.org/patchwork/patch/539137/
> 
> The fact that a CPU is part of lowest_mask but running a higher prio RT
> task means there is a race. Should we retry one more time to see if we find
> another CPU?

Isn't this what I did in v1?

https://lore.kernel.org/lkml/20200214163949.27850-4-qais.yousef@arm.com/

Thanks

--
Qais Yousef