linux-kernel - Re: [PATCH] sched/fair: Prevent dead task groups from regaining cfs

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Message-ID: <YYVpm7LRWMZMzxId@hirez.programming.kicks-ass.net>
Date:   Fri, 5 Nov 2021 18:27:55 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathias Krause <minipli@...ecurity.net>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Michal Koutný <mkoutny@...e.com>,
        Benjamin Segall <bsegall@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <Valentin.Schneider@....com>,
        linux-kernel@...r.kernel.org, Odin Ugedal <odin@...d.al>,
        Kevin Tanguy <kevin.tanguy@...p.ovh.com>,
        Brad Spengler <spender@...ecurity.net>
Subject: Re: [PATCH] sched/fair: Prevent dead task groups from regaining
 cfs_rq's

On Fri, Nov 05, 2021 at 06:14:33PM +0100, Mathias Krause wrote:
> Am 05.11.21 um 17:58 schrieb Peter Zijlstra:
> > On Fri, Nov 05, 2021 at 05:29:14PM +0100, Mathias Krause wrote:
> >>> Looks like it needs to be the kfree_rcu() one in this case. I'll prepare
> >>> a patch.
> >>
> >> Testing the below patch right now. Looking good so far. Will prepare a
> >> proper patch later, if we all can agree that this covers all cases.
> >>
> >> But the basic idea is to defer the kfree()'s to after the next RCU GP,
> >> which also means we need to free the tg object itself later. Slightly
> >> ugly. :/
> > 
> > Can't we add an rcu_head to struct task_group and simply call_rcu() the
> > thing with a free function?
> 
> There is already one and this patch makes use of it for the second RCU
> GP requirement. Basically, the patch is doing what you request, no? See
> the new free_fair_sched_group().

For some reason I thought you still did kfree_rcu(), I suppose reading
is hard. I'll give it another go after dinner.