Re: [PATCH v3] lockdep: Allow tuning tracing capacity constants.

[Tetsuo Handa]

On 2020/11/20 18:27, Dmitry Vyukov wrote:
> Peter, so far it looks like just a very large, but normal graph to me.
> The cheapest from an engineering point of view solution would be just
> to increase the constants. I assume a 2x increase should buy us lots
> of time to overflow.
> I can think of more elaborate solutions, e.g. using bitmasks to
> represent hot leaf and top-level locks. But it will both increase the
> resulting code complexity (no uniform representation anymore, all code
> will need to deal with different representations) and require some
> time investments (that I can't justify for me at least as compared to
> just throwing a bit more machine memory at it). And in the end it
> won't really reduce the size of the graph.
> What do you think?
> 

Yes, I think it is a normal graph; simply syzkaller kernels tend to record
a few times more dependencies than my idle kernel (shown bottom).

Peter, you guessed that the culprit is sysfs at
https://lkml.kernel.org/r/20200916115057.GO2674@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx , but
syzbot reported at https://syzkaller.appspot.com/text?tag=MachineInfo&x=99b8f2b092d9714f
that "BUG: MAX_LOCKDEP_ENTRIES too low!" can occur on a VM with only 2 CPUs.
Is your guess catching the culprit?

We could improve a few locks, but as a whole we won't be able to afford keeping
sum of individual dependencies under current threshold. Therefore, allow lockdep to
tune the capacity and allow syzkaller to dump when reaching the capacity will be
the way to go.



# cat /proc/lockdep_stats
 lock-classes:                         1236 [max: 8192]
 direct dependencies:                  9610 [max: 32768]
 indirect dependencies:               40401
 all direct dependencies:            174635
 dependency chains:                   11398 [max: 65536]
 dependency chain hlocks used:        42830 [max: 327680]
 dependency chain hlocks lost:            0
 in-hardirq chains:                      61
 in-softirq chains:                     414
 in-process chains:                   10923
 stack-trace entries:                 93041 [max: 524288]
 number of stack traces:               4997
 number of stack hash chains:          4292
 combined max dependencies:       281074520
 hardirq-safe locks:                     50
 hardirq-unsafe locks:                  805
 softirq-safe locks:                    146
 softirq-unsafe locks:                  722
 irq-safe locks:                        155
 irq-unsafe locks:                      805
 hardirq-read-safe locks:                 2
 hardirq-read-unsafe locks:             129
 softirq-read-safe locks:                11
 softirq-read-unsafe locks:             123
 irq-read-safe locks:                    11
 irq-read-unsafe locks:                 129
 uncategorized locks:                   224
 unused locks:                            0
 max locking depth:                      15
 max bfs queue depth:                   215
 chain lookup misses:                 11664
 chain lookup hits:                37393935
 cyclic checks:                       11053
 redundant checks:                        0
 redundant links:                         0
 find-mask forwards checks:            1588
 find-mask backwards checks:           1779
 hardirq on events:                17502380
 hardirq off events:               17502376
 redundant hardirq ons:                   0
 redundant hardirq offs:                  0
 softirq on events:                   90845
 softirq off events:                  90845
 redundant softirq ons:                   0
 redundant softirq offs:                  0
 debug_locks:                             1

 zapped classes:                          0
 zapped lock chains:                      0
 large chain blocks:                      1
# awk ' { if ($2 == "OPS:") print $5" "$9 } ' /proc/lockdep | sort -rV | head -n 30
423 (wq_completion)events
405 (wq_completion)events_unbound
393 &f->f_pos_lock
355 &p->lock
349 sb_writers#3
342 sb_writers#6
338 &of->mutex
330 (work_completion)(&entry->work)
330 pernet_ops_rwsem
289 epmutex
288 &ep->mtx
281 tty_mutex
280 &tty->legacy_mutex
273 &tty->legacy_mutex/1
269 &tty->ldisc_sem
268 (wq_completion)ipv6_addrconf
266 (work_completion)(&(&ifa->dad_work)->work)
266 (linkwatch_work).work
266 (addr_chk_work).work
266 &ldata->atomic_read_lock
265 (work_completion)(&buf->work)
265 rtnl_mutex
263 &tty->atomic_write_lock
262 &buf->lock
261 &tty->termios_rwsem
242 &port->buf.lock/1
242 kn->active#40
241 &o_tty->termios_rwsem/1
240 registration_lock
239 &ldata->output_lock
# awk ' { if ($2 == "OPS:") print $7" "$9 } ' /proc/lockdep | sort -rV | head -n 30
642 pool_lock
641 &obj_hash[i].lock
582 tk_core.seq.seqcount
561 hrtimer_bases.lock
560 &rt_rq->rt_runtime_lock
559 &rt_b->rt_runtime_lock
559 &cp->lock
559 &cfs_rq->removed.lock
559 &cfs_b->lock
558 &rq->lock
550 &tsk->delays->lock
549 &p->pi_lock
506 &base->lock
504 &n->list_lock
432 &____s->seqcount
404 &x->wait#10
401 &pool->lock
369 &zone->lock
330 rcu_node_0
326 &(kretprobe_table_locks[i].lock)
326 pgd_lock
324 &pgdat->lru_lock
323 lock#5
321 &page_wait_table[i]
319 ptlock_ptr(page)#2/1
318 ptlock_ptr(page)#2
316 &sem->wait_lock
316 &mm->page_table_lock
316 ptlock_ptr(page)
315 &anon_vma->rwsem