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

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Message-ID: <ZSCoLllO/cYnnIMi@chenyu5-mobl2.ccr.corp.intel.com>
Date:   Sat, 7 Oct 2023 08:37:02 +0800
From:   Chen Yu <yu.c.chen@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Mike Galbraith <efault@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Biju Das <biju.das.jz@...renesas.com>,
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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?

thanks,
Chenyu