Messages in this thread | | | Subject | Re: [PATCH] sched/fair: Prevent dead task groups from regaining cfs_rq's | From | Mathias Krause <> | Date | Fri, 5 Nov 2021 15:44:27 +0100 |
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Am 05.11.21 um 15:25 schrieb Vincent Guittot: > 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
Heh, indeed.
>> >> 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?
Looks like it needs to be the kfree_rcu() one in this case. I'll prepare a patch.
Thanks, Mathias
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