Messages in this thread | | | From | Vincent Guittot <> | Date | Fri, 25 Jan 2019 15:31:43 +0100 | Subject | Re: Crash in list_add_leaf_cfs_rq due to bad tmp_alone_branch |
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Hi Sargun,
On Mon, 21 Jan 2019 at 15:46, Vincent Guittot <vincent.guittot@linaro.org> wrote: > > Hi Sargun, > > Le Friday 18 Jan 2019 à 15:06:28 (+0100), Vincent Guittot a écrit : > > On Fri, 18 Jan 2019 at 11:16, Vincent Guittot > > <vincent.guittot@linaro.org> wrote: > > > > > > On Wed, 9 Jan 2019 at 23:43, Sargun Dhillon <sargun@sargun.me> wrote: > > > > > > > > On Wed, Jan 9, 2019 at 2:14 PM Sargun Dhillon <sargun@sargun.me> wrote: > > > > > > > > > > I picked up c40f7d74c741a907cfaeb73a7697081881c497d0 sched/fair: Fix > > > > > infinite loop in update_blocked_averages() by reverting a9e7f6544b9c > > > > > and put it on top of 4.19.13. In addition to this, I uninlined > > > > > list_add_leaf_cfs_rq for debugging. > > > > With the fix above applied, the code that manages the leaf_cfs_rq_list > > is the same since v4.9. > > Have you noticed similar problem on other older kernel version between > > v4.9 and v4.19 ? The problem might have been introduce while modifying > > other part of the scheduler like the sequence for adding/removing > > cgroup. > > > > Knowing the most recent kernel version without the problem could help > > to narrow the problem > > > > Thanks, > > Vincent > > > > > > > > > > > > This revealed a new bug that we didn't get to because we kept getting > > > > > crashes from the previous issue. When we are running with cgroups that > > > > > are rapidly changing, with CFS bandwidth control, and in addition > > > > > using the cpusets cgroup, we see this crash. Specifically, it seems to > > > > > occur with cgroups that are throttled and we change the allowed > > > > > cpuset. > > > > > > Thanks for the context, I will try to reproduce the problem and > > > understand how we can stop in the middle of walking to the > > > sched_entity branch with a parent not already added > > > > > > How many cgroup level have you got in you setup ? > > > > > > > > > > > > > > > > This patch from Gabriel should fix the problem: > > > > > > > > > > > > [PATCH] sched/fair: Reset tmp_alone_branch on cfs_rq delete > > > > > > > > When a child cfs_rq is added to the leaf cfs_rq list before its parent > > > > tmp_alone_branch is set to point to the child in preparation for the > > > > parent being added. > > > > > > > > If the child is deleted before the parent is added then tmp_alone_branch > > > > points to a freed cfs_rq. Any future reference to tmp_alone_branch will > > > > result in a use after free. > > > > > > So, the patch below is a temporary fix that helps to recover from the > > > situation where tmp_alone_branch doesn't finished back to > > > rq->leaf_cfs_rq_list > > > But this situation should not happened at the beginning > > I have been able to reproduce the situation where tmp_alone_branch doesn't > point to rq->leaf_cfs_rq_list after enqueuing a task. > > Can you try the patch below which ensures all cfs_rq of a cgroup branch will > be added in the list even if throttled ?
Did you get a chance to test this patch ?
Regards, Vincent
> > The algorithm used to order cfs_rq in rq->leaf_cfs_rq_list assumes that > it will walk down to root the 1st time a cfs_rq is used and we will finished > to add either a cfs_rq without parent or a cfs_rq with a parent that is already > on the list. But this is not always true in presence of throttling. > Because a cfs_rq can be throttled even if it has never been used but other CPUS > of the cgroup have already used all the bandwdith, we are not sure to go down to > the root and add all cfs_rq in the list. > > Ensure that all cfs_rq will be added in the list even if they are throttled. > > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > --- > kernel/sched/fair.c | 17 +++++++++++++++++ > 1 file changed, 17 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 6483834..ae468ab 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -352,6 +352,20 @@ static inline void list_del_leaf_cfs_rq(struct cfs_rq *cfs_rq) > } > } > > +static inline void list_add_branch_cfs_rq(struct sched_entity *se, struct rq *rq) > +{ > +struct cfs_rq *cfs_rq; > + > + for_each_sched_entity(se) { > + cfs_rq = cfs_rq_of(se); > + list_add_leaf_cfs_rq(cfs_rq); > + > + /* If parent is already in the list, we can stop */ > + if (rq->tmp_alone_branch == &rq->leaf_cfs_rq_list) > + break; > + } > +} > + > /* Iterate through all leaf cfs_rq's on a runqueue: */ > #define for_each_leaf_cfs_rq(rq, cfs_rq) \ > list_for_each_entry_rcu(cfs_rq, &rq->leaf_cfs_rq_list, leaf_cfs_rq_list) > @@ -5177,6 +5191,9 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags) > > } > > + /* Ensure that all cfs_rq have been added to the list */ > + list_add_branch_cfs_rq(se, rq); > + > hrtick_update(rq); > } > > > > > > > > > > > > > > > > > Signed-off-by: Gabriel Hartmann <gabriel.hartmann@gmail.com> > > > > Reported-by: Sargun Dhillon <sargun@sargun.me> > > > > --- > > > > kernel/sched/fair.c | 5 +++++ > > > > 1 file changed, 5 insertions(+) > > > > > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > > > index 7137bc343b4a..0987629cbb76 100644 > > > > --- a/kernel/sched/fair.c > > > > +++ b/kernel/sched/fair.c > > > > @@ -347,6 +347,11 @@ static inline void list_add_leaf_cfs_rq(struct > > > > cfs_rq *cfs_rq) > > > > static inline void list_del_leaf_cfs_rq(struct cfs_rq *cfs_rq) > > > > { > > > > if (cfs_rq->on_list) { > > > > + struct rq *rq = rq_of(cfs_rq); > > > > + > > > > + if (rq->tmp_alone_branch == &cfs_rq->leaf_cfs_rq_list) > > > > + rq->tmp_alone_branch = &rq->leaf_cfs_rq_list; > > > > + > > > > list_del_rcu(&cfs_rq->leaf_cfs_rq_list); > > > > cfs_rq->on_list = 0; > > > > }
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