From: Vincent Guittot <vincent.guittot@linaro.org>
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>,
"Michal Koutný" <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>
Subject: Re: [PATCH] sched/fair: Prevent dead task groups from regaining cfs_rq's
Date: Fri, 5 Nov 2021 15:25:46 +0100 [thread overview]
Message-ID: <CAKfTPtDthksitm02sLowDMKbWZ29efth-YcPi0zVSFqbaZfiMA@mail.gmail.com> (raw)
In-Reply-To: <cd3778d3-6980-a804-47e3-82b09dc960a4@grsecurity.net>
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
next prev parent reply other threads:[~2021-11-05 14:26 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-11 17:22 [PATCH] sched/fair: Use rq->lock when checking cfs_rq list presence Michal Koutný
2021-10-11 19:12 ` Odin Ugedal
2021-10-12 18:32 ` Tao Zhou
2021-10-13 18:52 ` Odin Ugedal
2021-10-13 14:39 ` Michal Koutný
2021-10-13 18:45 ` Odin Ugedal
2021-10-13 7:57 ` Vincent Guittot
2021-10-13 14:26 ` Michal Koutný
2021-11-02 16:02 ` task_group unthrottling and removal race (was Re: [PATCH] sched/fair: Use rq->lock when checking cfs_rq list) presence Michal Koutný
2021-11-02 20:20 ` Odin Ugedal
2021-11-03 9:51 ` Mathias Krause
2021-11-03 10:51 ` Mathias Krause
2021-11-03 11:10 ` Michal Koutný
2021-11-03 14:16 ` Mathias Krause
2021-11-03 19:06 ` [PATCH] sched/fair: Prevent dead task groups from regaining cfs_rq's Mathias Krause
2021-11-03 22:03 ` Benjamin Segall
2021-11-04 8:50 ` Vincent Guittot
2021-11-04 15:13 ` Mathias Krause
2021-11-04 16:49 ` Vincent Guittot
2021-11-04 17:37 ` Mathias Krause
2021-11-05 14:25 ` Vincent Guittot [this message]
2021-11-05 14:44 ` Mathias Krause
2021-11-05 16:29 ` Mathias Krause
2021-11-05 16:58 ` Peter Zijlstra
2021-11-05 17:14 ` Mathias Krause
2021-11-05 17:27 ` Peter Zijlstra
2021-11-05 17:40 ` Mathias Krause
2021-11-06 10:48 ` Peter Zijlstra
2021-11-08 10:27 ` Mathias Krause
2021-11-08 11:40 ` Peter Zijlstra
2021-11-08 15:06 ` Mathias Krause
2021-11-10 15:14 ` Vincent Guittot
2021-11-09 18:47 ` Michal Koutný
2021-11-10 15:17 ` Vincent Guittot
2021-11-04 20:46 ` Benjamin Segall
2021-11-04 18:49 ` Michal Koutný
2021-11-05 14:55 ` Mathias Krause
2021-11-05 14:58 ` Peter Zijlstra
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