There are nested loops on debug objects free path, sometimes it may take over hundred thousands of loops, then cause soft lockup with !CONFIG_PREEMPT occasionally, like below:
NMI watchdog: BUG: soft lockup - CPU#15 stuck for 22s! [stress-ng-getde:110342] CPU: 15 PID: 110342 Comm: stress-ng-getde Tainted: G E 4.9.44-003.ali3000.alios7.x86_64.debug #1 Hardware name: Dell Inc. PowerEdge R720xd/0X6FFV, BIOS 1.6.0 03/07/2013 task: ffff884cbb0d0000 task.stack: ffff884cabc70000 RIP: 0010:[<ffffffff817d647b>] [<ffffffff817d647b>] _raw_spin_unlock_irqrestore+0x3b/0x60 RSP: 0018:ffff884cabc77b78 EFLAGS: 00000292 RAX: ffff884cbb0d0000 RBX: 0000000000000292 RCX: 0000000000000000 RDX: ffff884cbb0d0000 RSI: 0000000000000001 RDI: 0000000000000292 RBP: ffff884cabc77b88 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000000000001 R11: 0000000000000001 R12: ffffffff8357a0d8 R13: ffff884cabc77bc8 R14: ffffffff8357a0d0 R15: 00000000000000fc FS: 00002aee845fd2c0(0000) GS:ffff8852bd400000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000002991808 CR3: 0000005123abf000 CR4: 00000000000406e0 Stack: ffff884ff4fe0000 ffff884ff4fd8000 ffff884cabc77c00 ffffffff8141177e 0000000000000202 ffff884cbb0d0000 ffff884cabc77bc8 0000000000000006 ffff884ff4fda000 ffffffff8357a0d8 0000000000000000 91f5d976f6020b6c Call Trace: [<ffffffff8141177e>] debug_check_no_obj_freed+0x13e/0x220 [<ffffffff811f8751>] __free_pages_ok+0x1f1/0x5c0 [<ffffffff811fa785>] __free_pages+0x25/0x40 [<ffffffff812638db>] __free_slab+0x19b/0x270 [<ffffffff812639e9>] discard_slab+0x39/0x50 [<ffffffff812679f7>] __slab_free+0x207/0x270 [<ffffffff81269966>] ___cache_free+0xa6/0xb0 [<ffffffff8126c267>] qlist_free_all+0x47/0x80 [<ffffffff8126c5a9>] quarantine_reduce+0x159/0x190 [<ffffffff8126b3bf>] kasan_kmalloc+0xaf/0xc0 [<ffffffff8126b8a2>] kasan_slab_alloc+0x12/0x20 [<ffffffff81265e8a>] kmem_cache_alloc+0xfa/0x360 [<ffffffff812abc8f>] ? getname_flags+0x4f/0x1f0 [<ffffffff812abc8f>] getname_flags+0x4f/0x1f0 [<ffffffff812abe42>] getname+0x12/0x20 [<ffffffff81298da9>] do_sys_open+0xf9/0x210 [<ffffffff81298ede>] SyS_open+0x1e/0x20 [<ffffffff817d6e01>] entry_SYSCALL_64_fastpath+0x1f/0xc2 The code path might be called in either atomic or non-atomic context, and in_atomic() can't tell if current context is atomic or not on !PREEMPT kernel, so cond_resched() can't be used to prevent from the softlockup. Defer objects free outside of the loop in a batch to save some cycles in the loop. The objects will be added to a global free list, then put them back to pool list in a work. When allocating objects, check if there are any objects available on the global free list and just reuse the objects if the global free list is not empty. Reuse pool lock to protect the free list. Using a new lock sounds overkilling. Export the number of objects on the global freelist to sysfs, which looks like: max_chain :79 max_loops :8147 warnings :0 fixups :0 pool_free :1697 pool_min_free :346 pool_used :15356 pool_max_used :23933 on_free_list :39 objs_allocated:32617 objs_freed :16588 Signed-off-by: Yang Shi <[email protected]> Suggested-by: Thomas Gleixner <[email protected]> Cc: Waiman Long <[email protected]> --- v4: Dropped touching softlockup watchdog approach, and defer objects free outside the for loop per the suggestion from tglx. lib/debugobjects.c | 48 +++++++++++++++++++++++++++++++++++++++--------- 1 file changed, 39 insertions(+), 9 deletions(-) diff --git a/lib/debugobjects.c b/lib/debugobjects.c index 166488d..cebf952 100644 --- a/lib/debugobjects.c +++ b/lib/debugobjects.c @@ -42,11 +42,14 @@ struct debug_bucket { static DEFINE_RAW_SPINLOCK(pool_lock); static HLIST_HEAD(obj_pool); +static HLIST_HEAD(obj_to_free); static int obj_pool_min_free = ODEBUG_POOL_SIZE; static int obj_pool_free = ODEBUG_POOL_SIZE; static int obj_pool_used; static int obj_pool_max_used; +/* The number of objs on the global free list */ +static int obj_free; static struct kmem_cache *obj_cache; static int debug_objects_maxchain __read_mostly; @@ -141,7 +144,8 @@ static struct debug_obj *lookup_object(void *addr, struct debug_bucket *b) } /* - * Allocate a new object. If the pool is empty, switch off the debugger. + * Allocate a new object. Retrieve from global freelist first. If the pool is + * empty, switch off the debugger. * Must be called with interrupts disabled. */ static struct debug_obj * @@ -150,6 +154,13 @@ static struct debug_obj *lookup_object(void *addr, struct debug_bucket *b) struct debug_obj *obj = NULL; raw_spin_lock(&pool_lock); + if (obj_free > 0 && (obj_pool_free < obj_pool_min_free)) { + obj = hlist_entry(obj_to_free.first, typeof(*obj), node); + obj_free--; + hlist_del(&obj->node); + goto out; + } + if (obj_pool.first) { obj = hlist_entry(obj_pool.first, typeof(*obj), node); @@ -169,6 +180,8 @@ static struct debug_obj *lookup_object(void *addr, struct debug_bucket *b) if (obj_pool_free < obj_pool_min_free) obj_pool_min_free = obj_pool_free; } + +out: raw_spin_unlock(&pool_lock); return obj; @@ -186,11 +199,25 @@ static struct debug_obj *lookup_object(void *addr, struct debug_bucket *b) static void free_obj_work(struct work_struct *work) { struct debug_obj *objs[ODEBUG_FREE_BATCH]; + struct hlist_node *tmp; + struct debug_obj *obj; unsigned long flags; int i; if (!raw_spin_trylock_irqsave(&pool_lock, flags)) return; + + /* Move free obj to pool list from global free list */ + if (obj_free > 0) { + hlist_for_each_entry_safe(obj, tmp, &obj_to_free, node) { + hlist_del(&obj->node); + hlist_add_head(&obj->node, &obj_pool); + obj_pool_free++; + obj_pool_used--; + obj_free--; + } + } + while (obj_pool_free >= debug_objects_pool_size + ODEBUG_FREE_BATCH) { for (i = 0; i < ODEBUG_FREE_BATCH; i++) { objs[i] = hlist_entry(obj_pool.first, @@ -716,7 +743,6 @@ static void __debug_check_no_obj_freed(const void *address, unsigned long size) { unsigned long flags, oaddr, saddr, eaddr, paddr, chunks; struct hlist_node *tmp; - HLIST_HEAD(freelist); struct debug_obj_descr *descr; enum debug_obj_state state; struct debug_bucket *db; @@ -752,18 +778,17 @@ static void __debug_check_no_obj_freed(const void *address, unsigned long size) goto repeat; default: hlist_del(&obj->node); - hlist_add_head(&obj->node, &freelist); + /* Put obj on the global free list */ + raw_spin_lock(&pool_lock); + hlist_add_head(&obj->node, &obj_to_free); + /* Update the counter of objs on the global freelist */ + obj_free++; + raw_spin_unlock(&pool_lock); break; } } raw_spin_unlock_irqrestore(&db->lock, flags); - /* Now free them */ - hlist_for_each_entry_safe(obj, tmp, &freelist, node) { - hlist_del(&obj->node); - free_object(obj); - } - if (cnt > debug_objects_maxchain) debug_objects_maxchain = cnt; @@ -772,6 +797,10 @@ static void __debug_check_no_obj_freed(const void *address, unsigned long size) if (max_loops > debug_objects_maxloops) debug_objects_maxloops = max_loops; + + /* Schedule work to move free objs to pool list */ + if (obj_free > 0) + schedule_work(&debug_obj_work); } void debug_check_no_obj_freed(const void *address, unsigned long size) @@ -793,6 +822,7 @@ static int debug_stats_show(struct seq_file *m, void *v) seq_printf(m, "pool_min_free :%d\n", obj_pool_min_free); seq_printf(m, "pool_used :%d\n", obj_pool_used); seq_printf(m, "pool_max_used :%d\n", obj_pool_max_used); + seq_printf(m, "on_free_list :%d\n", obj_free); seq_printf(m, "objs_allocated:%d\n", debug_objects_allocated); seq_printf(m, "objs_freed :%d\n", debug_objects_freed); return 0; -- 1.8.3.1
