linux-kernel - Re: [PATCH] sched/fair: fix pick_eevdf to always find the correct se

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Date:   Tue, 10 Oct 2023 10:08:31 +0800
From:   Chen Yu <yu.c.chen@...el.com>
To:     Phil Auld <pauld@...hat.com>
CC:     Biju Das <biju.das.jz@...renesas.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mike Galbraith <efault@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        "bsegall@...gle.com" <bsegall@...gle.com>,
        "bristot@...hat.com" <bristot@...hat.com>,
        "chris.hyser@...cle.com" <chris.hyser@...cle.com>,
        "corbet@....net" <corbet@....net>,
        "dietmar.eggemann@....com" <dietmar.eggemann@....com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "joel@...lfernandes.org" <joel@...lfernandes.org>,
        "joshdon@...gle.com" <joshdon@...gle.com>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "kprateek.nayak@....com" <kprateek.nayak@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "patrick.bellasi@...bug.net" <patrick.bellasi@...bug.net>,
        Pavel Machek <pavel@....cz>, "pjt@...gle.com" <pjt@...gle.com>,
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        <linux-renesas-soc@...r.kernel.org>
Subject: Re: [PATCH] sched/fair: fix pick_eevdf to always find the correct se

Hi Phil,

On 2023-10-09 at 10:27:40 -0400, Phil Auld wrote:
> On Sat, Oct 07, 2023 at 02:42:10PM +0800 Chen Yu wrote:
> > Hi Biju,
> > 
> > On 2023-10-07 at 06:26:05 +0000, Biju Das wrote:
> > > Hi Chen Yu,
> > > 
> > > > Subject: Re: [PATCH] sched/fair: fix pick_eevdf to always find the correct
> > > > se
> > > > 
> > > > On 2023-10-06 at 21:24:45 +0200, Peter Zijlstra wrote:
> > > > > On Fri, Oct 06, 2023 at 05:55:01PM +0200, Peter Zijlstra wrote:
> > > > >
> > > > > > And yeah, min_deadline is hosed somehow.
> > > > > >
> > > > > > migration/28-185     [028] d..2.    70.264274: validate_cfs_rq: --- /
> > > > > > migration/28-185     [028] d..2.    70.264277: __print_se:
> > > > ffff88845cf48080 w: 1024 ve: -58857638 lag: 870381 vd: -55861854 vmd: -
> > > > 66302085 E (11372/tr)
> > > > > > migration/28-185     [028] d..2.    70.264280: __print_se:
> > > > ffff88810d165800 w: 25 ve: -80323686 lag: 22336429 vd: -41496434 vmd: -
> > > > 66302085 E (-1//autogroup-31)
> > > > > > migration/28-185     [028] d..2.    70.264282: __print_se:
> > > > ffff888108379000 w: 25 ve: 0 lag: -57987257 vd: 114632828 vmd: 114632828 N
> > > > (-1//autogroup-33)
> > > > > > migration/28-185     [028] d..2.    70.264283: validate_cfs_rq:
> > > > min_deadline: -55861854 avg_vruntime: -62278313462 / 1074 = -57987256
> > > > > >
> > > > > > I need to go make dinner (kids hungry), but I'll see if I can figure
> > > > > > out how this happens...
> > > > >
> > > > > *sigh*, does the below help?
> > > > >
> > > > > ---
> > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index
> > > > > 04fbcbda97d5..6a670f119efa 100644
> > > > > --- a/kernel/sched/fair.c
> > > > > +++ b/kernel/sched/fair.c
> > > > > @@ -3632,6 +3747,7 @@ static void reweight_entity(struct cfs_rq *cfs_rq,
> > > > struct sched_entity *se,
> > > > >  		 */
> > > > >  		deadline = div_s64(deadline * old_weight, weight);
> > > > >  		se->deadline = se->vruntime + deadline;
> > > > > +		min_deadline_cb_propagate(&se->run_node, NULL);
> > > > >  	}
> > > > >
> > > > >  #ifdef CONFIG_SMP
> > > > 
> > > > Is it because without this patch, the se->deadline is not always synced
> > > > with se->min_deadline, then in pick_eevdf() the following condition could
> > > > not be met, thus we get a NULL candidate and has to pick the leftmost one?
> > > > 	if (se->deadline == se->min_deadline)
> > > > 
> > > > Regarding the circular locking warning triggered by printk, does it mean we
> > > > should not get a NULL candidate from __pick_eevdf() in theory? And besides,
> > > > we should not use printk with rq lock hold?
> > > 
> > > Is it not a useful error log? At least from the initial report Marek Szyprowski doesn't see "EEVDF scheduling fail, picking leftmost" but seen only warning triggered by this in the logs.
> > > 
> > 
> > Yes, it is a useful log. I was trying to figure out the safe scenario to use
> > printk.
> >
> 
> You should not use printk with a rq lock held as some console implementations
> (framebuffer etc) call schedule_work() which loops back into the scheduler
> and bad things happen.
> 
> Some places in the scheduler use printk_deferred() when needed. 
> 
> I think this will be better when the RT printk stuff is fully merged. 
>

I see, got it! Thanks for the education.

thanks,
Chenyu