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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Fri, 5 Nov 2021 15:25:46 +0100
Message-ID: <CAKfTPtDthksitm02sLowDMKbWZ29efth-YcPi0zVSFqbaZfiMA@mail.gmail.com>
Subject: Re: [PATCH] sched/fair: Prevent dead task groups from regaining cfs_rq's
To:     Mathias Krause <minipli@grsecurity.net>
Cc:     Benjamin Segall <bsegall@google.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        =?UTF-8?Q?Michal_Koutn=C3=BD?= <mkoutny@suse.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <Valentin.Schneider@arm.com>,
        linux-kernel@vger.kernel.org, Odin Ugedal <odin@uged.al>,
        Kevin Tanguy <kevin.tanguy@corp.ovh.com>,
        Brad Spengler <spender@grsecurity.net>
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On Thu, 4 Nov 2021 at 18:37, Mathias Krause <minipli@grsecurity.net> wrote:
>
> Am 04.11.21 um 17:49 schrieb Vincent Guittot:
> > [snip]
> >
> > Ok so we must have 2 GPs:
> >
> > list_del_rcu(&tg->siblings);
> > GP to wait for the end of ongoing walk_tg_tree_from : synchronize_rcu
> > in your patch
> > list_del_leaf_cfs_rq(tg->cfs_rq[cpu]); if on_list
> > remove_entity_load_avg(tg->se[cpu]);
> > GP to wait for the end of ongoing for_each_leaf_cfs_rq_safe (print_cfs_stats)
> > kfree everything
>
> Basically yes, but with my patch we already have these two, as there's
> at least one RCU GP between after sched_offline_group() finishes and
> sched_free_group() / cpu_cgroup_css_free() starts.
>
> So we either use my patch as-is or move unregister_fair_sched_group() to
> free_fair_sched_group() and use kfree_rcu() instead of kfree(). Both
> approaches have pros and cons.
>
> Pro for my version is the early unlinking of cfs_rq's for dead task
> groups, so no surprises later on. Con is the explicit synchronize_rcu().

which blocks the caller and could be problematic

It seems that LKP has reported such issue:
20211104145128.GC6499@xsang-OptiPlex-9020

>
> Pro for the kfree_rcu() approach is the lack of the explicit
> synchronize_rcu() call, so no explicit blocking operation. Con is that
> we have cfs_rq's re-added to dead task groups which feels wrong and need
> to find a suitable member to overlap with the rcu_head in each involved
> data type.
>
> Which one do you prefer?
>
> Thanks,
> Mathias