Hi Ingo, I got the following message and the kernel crashed when testing 2.6.24.2-rt2. Unable to handle kernel NULL pointer dereference at 0000000000000128 RIP: [<ffffffff80229805>] pick_next_task_fair+0x2d/0x42 PGD 211db1067 PUD 211c1d067 PMD 0 Oops: 0000 [1] PREEMPT SMP CPU 2 Modules linked in: Pid: 898, comm: stress Not tainted 2.6.24.2-rt2 #1 RIP: 0010:[<ffffffff80229805>] [<ffffffff80229805>] pick_next_task_fair+0x2d/0x42 RSP: 0018:ffff8101ac423948 EFLAGS: 00010046 RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff81022e562300 RDX: ffff8101ac4239b0 RSI: ffff81022e562300 RDI: ffff8100050196e0 RBP: ffff8101ac423958 R08: ffff810005015680 R09: ffff810005015800 R10: 0000000000000000 R11: 0000000000000001 R12: ffff810005011680 R13: 0000000000000001 R14: ffff810005019680 R15: 00000001002f7a43 FS: 00002ada5c132b00(0000) GS:ffff81022fc057c0(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b CR2: 0000000000000128 CR3: 0000000211dfc000 CR4: 00000000000006e0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400 Process stress (pid: 898, threadinfo ffff8101ac422000, task ffff81022e562300) Stack: ffffffff802292e4 ffff810005015ac0 ffff8101ac4239e8 ffffffff804d9685 ffff8101ac4239b0 ffffffff8022f2bd 00000002ac423998 ffff81022e562300 ffff8101ac4239a8 ffff81022e562640 00000000000000ff ffffffff804db527 Call Trace: [<ffffffff802292e4>] put_prev_task_rt+0xd/0x18 [<ffffffff804d9685>] __schedule+0x414/0x771 [<ffffffff8022f2bd>] add_preempt_count+0x18/0xb2 [<ffffffff804db527>] __spin_unlock+0x14/0x2e [<ffffffff804d9cdc>] schedule+0xdf/0xff [<ffffffff804da557>] rt_spin_lock_slowlock+0xf9/0x19e [<ffffffff804daed8>] __rt_spin_lock+0x6b/0x70 [<ffffffff804daee6>] rt_spin_lock+0x9/0xb [<ffffffff8027b57f>] page_lock_anon_vma+0x2b/0x3b [<ffffffff8027c38c>] page_referenced+0x49/0xf5 [<ffffffff8026f40b>] shrink_active_list+0x222/0x563 [<ffffffff8022f2bd>] add_preempt_count+0x18/0xb2 [<ffffffff802705d5>] shrink_zone+0xcc/0x10f [<ffffffff80271139>] try_to_free_pages+0x183/0x27a [<ffffffff8026b0da>] __alloc_pages+0x1fb/0x344 [<ffffffff8027c5ca>] anon_vma_prepare+0x29/0xf9 [<ffffffff80274fc8>] handle_mm_fault+0x251/0x700 [<ffffffff8020c7e6>] retint_kernel+0x26/0x30 [<ffffffff8021f522>] do_page_fault+0x315/0x6bf [<ffffffff804db527>] __spin_unlock+0x14/0x2e [<ffffffff8022f1af>] finish_task_switch+0x2b/0x90 [<ffffffff804db7c9>] error_exit+0x0/0x51 Code: 48 8b bb 28 01 00 00 48 85 ff 75 dd 48 8d 43 b8 41 58 5b 5d I've gotten a kernel dump at this time. The backtrace says; (gdb) bt #0 pick_next_task_fair (rq=<value optimized out>) at kernel/sched_fair.c:680 #1 0xffffffff804d9685 in __schedule () at kernel/sched.c:3783 #2 0xffffffff804d9cdc in schedule () at kernel/sched.c:3914 #3 0xffffffff804da557 in rt_spin_lock_slowlock (lock=0xffff81022e8ca618) at kernel/rtmutex.c:735 #4 0xffffffff804daed8 in __rt_spin_lock (lock=0xffff81022e8ca618) at kernel/rtmutex.c:646 #5 0xffffffff804daee6 in rt_spin_lock (lock=0xffff8100050196e0) at kernel/rtmutex.c:799 #6 0xffffffff8027b57f in page_lock_anon_vma (page=<value optimized out>) at mm/rmap.c:172 #7 0xffffffff8027c38c in page_referenced (page=0xffff8100050196e0, is_locked=777396992) at mm/rmap.c:309 (snip) and here is the kernel/sched_fair.c:680 static inline struct cfs_rq *group_cfs_rq(struct sched_entity *grp) { return grp->my_q; } It seems that grp is NULL, so I think pick_next_entity() returns NULL. I think pick_next_entity() could return NULL when first_fair(cfs_rq) is false. static struct sched_entity *pick_next_entity(struct cfs_rq *cfs_rq) { struct sched_entity *se = NULL; if (first_fair(cfs_rq)) { se = __pick_next_entity(cfs_rq); set_next_entity(cfs_rq, se); } return se; } static struct task_struct *pick_next_task_fair(struct rq *rq) { struct cfs_rq *cfs_rq = &rq->cfs; struct sched_entity *se; if (unlikely(!cfs_rq->nr_running)) return NULL; do { se = pick_next_entity(cfs_rq); cfs_rq = group_cfs_rq(se); } while (cfs_rq); return task_of(se); } I'm not sure the actual scenario to cause this panic. I also don't know how to reproduce it, I was running some benchmarks at the week end and found it the Monday morning. I've encountered it on 2.6.24.2-rt2, but 2.6.24.2 and 2.6.25-rc3 also have the same code pick_next_task_fair(). However latest sched-devel git tree has a modified code. I'm not so familiar the CFS, is this investigation correct? Thanks, Hiroshi Shimamoto - To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html