lkml.org 
[lkml]   [2019]   [Jan]   [30]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH v2] sched/fair: Fix insertion in rq->leaf_cfs_rq_list
On Wed, 30 Jan 2019 at 15:01, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Wed, Jan 30, 2019 at 06:22:47AM +0100, Vincent Guittot wrote:
> > Sargun reported a crash:
> > "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.
> >
> > 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."
> >
> > 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 finish
> > 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.
> >
> > Reported-by: Sargun Dhillon <sargun@sargun.me>
> > Fixes: 9c2791f936ef ("Fix hierarchical order in rq->leaf_cfs_rq_list")
> > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
>
> Given my previous patch; how about something like so instead?

I still have to run some tests but this looks good to me

>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -282,13 +282,15 @@ static inline struct cfs_rq *group_cfs_r
> return grp->my_q;
> }
>
> -static inline void list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
> +static inline bool list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
> {
> struct rq *rq = rq_of(cfs_rq);
> int cpu = cpu_of(rq);
>
> if (cfs_rq->on_list)
> - return;
> + return rq->tmp_alone_branch == &rq->leaf_cfs_rq_list;
> +
> + cfs_rq->on_list = 1;
>
> /*
> * Ensure we either appear before our parent (if already
> @@ -315,7 +317,10 @@ static inline void list_add_leaf_cfs_rq(
> * list.
> */
> rq->tmp_alone_branch = &rq->leaf_cfs_rq_list;
> - } else if (!cfs_rq->tg->parent) {
> + return true;
> + }
> +
> + if (!cfs_rq->tg->parent) {
> /*
> * cfs rq without parent should be put
> * at the tail of the list.
> @@ -327,23 +332,22 @@ static inline void list_add_leaf_cfs_rq(
> * tmp_alone_branch to the beginning of the list.
> */
> rq->tmp_alone_branch = &rq->leaf_cfs_rq_list;
> - } else {
> - /*
> - * The parent has not already been added so we want to
> - * make sure that it will be put after us.
> - * tmp_alone_branch points to the begin of the branch
> - * where we will add parent.
> - */
> - list_add_rcu(&cfs_rq->leaf_cfs_rq_list,
> - rq->tmp_alone_branch);
> - /*
> - * update tmp_alone_branch to points to the new begin
> - * of the branch
> - */
> - rq->tmp_alone_branch = &cfs_rq->leaf_cfs_rq_list;
> + return true;
> }
>
> - cfs_rq->on_list = 1;
> + /*
> + * The parent has not already been added so we want to
> + * make sure that it will be put after us.
> + * tmp_alone_branch points to the begin of the branch
> + * where we will add parent.
> + */
> + list_add_rcu(&cfs_rq->leaf_cfs_rq_list, rq->tmp_alone_branch);
> + /*
> + * update tmp_alone_branch to points to the new begin
> + * of the branch
> + */
> + rq->tmp_alone_branch = &cfs_rq->leaf_cfs_rq_list;
> + return false;
> }
>
> static inline void list_del_leaf_cfs_rq(struct cfs_rq *cfs_rq)
> @@ -4999,6 +5003,12 @@ static void __maybe_unused unthrottle_of
> }
>
> #else /* CONFIG_CFS_BANDWIDTH */
> +
> +static inline bool cfs_bandwidth_used(void)
> +{
> + return false;
> +}
> +
> static inline u64 cfs_rq_clock_task(struct cfs_rq *cfs_rq)
> {
> return rq_clock_task(rq_of(cfs_rq));
> @@ -5190,6 +5200,21 @@ enqueue_task_fair(struct rq *rq, struct
>
> }
>
> + if (cfs_bandwidth_used()) {
> + /*
> + * When bandwidth control is enabled; the cfs_rq_throttled()
> + * breaks in the above iteration can result in incomplete
> + * leaf list maintenance, resulting in triggering the assertion
> + * below.
> + */
> + for_each_sched_entity(se) {
> + cfs_rq = cfs_rq_of(se);
> +
> + if (list_add_leaf_cfs_rq(cfs_rq))
> + break;
> + }
> + }
> +
> assert_list_leaf_cfs_rq(rq);
>
> hrtick_update(rq);

\
 
 \ /
  Last update: 2019-01-30 15:30    [W:0.079 / U:2.924 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site