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SubjectRe: [BUG] cpu hotplug vs scheduler
Hi,

> [ ... ]
>
> [4298303.713901] Call Trace:
> [4298303.713901] [<ffffffff804373fe>] schedule+0x414/0x6ab
> [4298303.713901] [<ffffffff8023060a>] ? hrtick_set+0x9d/0xe8
> [4298303.713901] [<ffffffff8043772f>] ? thread_return+0x9a/0xbf
> [4298303.713901] [<ffffffff80231652>] migration_thread+0x185/0x22d
> [4298303.713901] [<ffffffff802314cd>] ? migration_thread+0x0/0x22d
> [4298303.713901] [<ffffffff8024afe6>] kthread+0x49/0x77
> [4298303.713901] [<ffffffff8020d228>] child_rip+0xa/0x12
> [4298303.713901] [<ffffffff8024af9d>] ? kthread+0x0/0x77
> [4298303.713901] [<ffffffff8020d21e>] ? child_rip+0x0/0x12
> [4298303.713901]
> [4298303.713901]
> [4298303.713901] Code: c0 74 28 48 8b 7b 58 4c 8d 60 f0 48 85 ff 74 10 4c
> 89 e6 e8 df cc ff ff 85 c0 75 04 4c 8b 63 58 4c 89 e6 48 89 df e8 4a e5 ff
> ff <49> 8b 9c 24 58 01 00 00 48 85 db 75 bf 49 83 ec 38 4c 89 ef 4c
> [4298303.713901] RIP [<ffffffff8022e722>] pick_next_task_fair+0x55/0x7c
>
> This seems to be the assignment to cfs_rq after pick_next_entity().

[ cc'ed a few folks. ]


So the cfs-tree likely gets out-of-sync. I pressume, it won't be
reproducible with CONFIG_SCHED_GROUP options being disabled.

Anyway, would you try one of these debug-patches (not sure about the
workability of the second one though :-/)

Let's check what are the values for 'cfs_rq->weight.load/nr_running'.

thanks in advance,

(non-whitespace-damaged versions are enclosed)

---
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1291,6 +1291,12 @@ static struct task_struct
*pick_next_task_fair(struct rq *rq)

do {
se = pick_next_entity(cfs_rq);
+
+ if (unlikely(!se))
+ printk(KERN_ERR "BUG: se == NULL but
nr_running (%ld), load (%ld),"
+ " rq-nr_running (%ld), rq-load (%ld)\n",
+ cfs_rq->nr_running,
cfs_rq->load.weight, rq->nr_running, rq->load.weight);
+
cfs_rq = group_cfs_rq(se);
} while (cfs_rq);

---
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1280,6 +1280,8 @@ static void check_preempt_wakeup(struct rq *rq,
struct task_str uct *p)
resched_task(curr);
}

+static void sysrq_sched_debug_show(void);
+
static struct task_struct *pick_next_task_fair(struct rq *rq)
{
struct task_struct *p;
@@ -1291,6 +1293,10 @@ static struct task_struct
*pick_next_task_fair(struct rq *rq)

do {
se = pick_next_entity(cfs_rq);
+
+ if (unlikely(!se))
+ sysrq_sched_debug_show();
+
cfs_rq = group_cfs_rq(se);
} while (cfs_rq);

---
--
Best regards,
Dmitry Adamushko
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index e24ecd3..1dcc470 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1280,6 +1280,8 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p)
resched_task(curr);
}

+static void sysrq_sched_debug_show(void);
+
static struct task_struct *pick_next_task_fair(struct rq *rq)
{
struct task_struct *p;
@@ -1291,6 +1293,10 @@ static struct task_struct *pick_next_task_fair(struct rq *rq)

do {
se = pick_next_entity(cfs_rq);
+
+ if (unlikely(!se))
+ sysrq_sched_debug_show();
+
cfs_rq = group_cfs_rq(se);
} while (cfs_rq);
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index e24ecd3..e21e020 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1291,6 +1291,12 @@ static struct task_struct *pick_next_task_fair(struct rq *rq)

do {
se = pick_next_entity(cfs_rq);
+
+ if (unlikely(!se))
+ printk(KERN_ERR "BUG: se == NULL but nr_running (%ld), load (%ld),"
+ " rq-nr_running (%ld), rq-load (%ld)\n",
+ cfs_rq->nr_running, cfs_rq->load.weight, rq->nr_running, rq->load.weight);
+
cfs_rq = group_cfs_rq(se);
} while (cfs_rq);
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