Messages in this thread | | | From | Vincent Guittot <> | Date | Fri, 5 Nov 2021 15:25:46 +0100 | Subject | Re: [PATCH] sched/fair: Prevent dead task groups from regaining cfs_rq's |
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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
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