Re: [PATCH] slub: limit count of partial slabs scanned to gather statistics

Konstantin Khlebnikov
2020-05-07T05:15:53Z

On 07/05/2020 06.01, Qian Cai wrote:
>
>
>> On May 6, 2020, at 3:06 PM, Qian Cai <cai@lca.pw> wrote:
>>
>>
>>
>>> On May 4, 2020, at 12:07 PM, Konstantin Khlebnikov <khlebnikov@yandex-team.ru> wrote:
>>>
>>> To get exact count of free and used objects slub have to scan list of
>>> partial slabs. This may take at long time. Scanning holds spinlock and
>>> blocks allocations which move partial slabs to per-cpu lists and back.
>>>
>>> Example found in the wild:
>>>
>>> # cat /sys/kernel/slab/dentry/partial
>>> 14478538 N0=7329569 N1=7148969
>>> # time cat /sys/kernel/slab/dentry/objects
>>> 286225471 N0=136967768 N1=149257703
>>>
>>> real 0m1.722s
>>> user 0m0.001s
>>> sys 0m1.721s
>>>
>>> The same problem in slab was addressed in commit f728b0a5d72a ("mm, slab:
>>> faster active and free stats") by adding more kmem cache statistics.
>>> For slub same approach requires atomic op on fast path when object frees.
>>>
>>> Let's simply limit count of scanned slabs and print warning.
>>> Limit set in /sys/module/slub/parameters/max_partial_to_count.
>>> Default is 10000 which should be enough for most sane cases.
>>>
>>> Return linear approximation if list of partials is longer than limit.
>>> Nobody should notice difference.
>>>
>>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
>>
>> This patch will trigger the warning under memory pressure, and then makes lockdep unhappy. Also, it is almost impossible tell how many max_partial_to_count is sufficient from user perspective.

Oops, my bad. Printing under this lock indeed a bad idea.

Probably it's better to simply remove this message.
I cannot imagine situation when precise count of object matters at such scale.

>
> Andrew, Stephen, can you remove this patch from linux-next?
>
> Even read some procfs files would trigger the warning and lockdep on a debug kernel probably due to kmemleak and debugobjects that would require more partial slabs objects. Thus, it would be problematic to break testing bots on linux-next like this.
>
>>
>> [ 6371.600511] SLUB: too much partial slabs to count all objects, increase max_partial_to_count.
>> [ 6371.601399] irq event stamp: 8132599
>>
>> [ 6371.611415] ======================================================
>> [ 6371.611417] WARNING: possible circular locking dependency detected
>> [ 6371.611419] 5.7.0-rc4-mm1+ #1 Not tainted
>> [ 6371.611421] ------------------------------------------------------
>> [ 6371.611423] oom02/43515 is trying to acquire lock:
>> [ 6371.611425] ffffffff893b8980 (console_owner){-.-.}-{0:0}, at: console_unlock+0x240/0x750
>>
>> [ 6371.611433] but task is already holding lock:
>> [ 6371.611434] ffff8886456fcb98 (&n->list_lock){-.-.}-{2:2}, at: count_partial+0x29/0xe0
>>
>> [ 6371.611441] which lock already depends on the new lock.
>>
>>
>> [ 6371.611445] the existing dependency chain (in reverse order) is:
>>
>> [ 6371.611446] -> #3 (&n->list_lock){-.-.}-{2:2}:
>> [ 6371.611452] _raw_spin_lock+0x2f/0x40
>> [ 6371.611453] deactivate_slab+0x37a/0x690
>> [ 6371.611455] ___slab_alloc+0x65d/0x810
>> [ 6371.611456] __slab_alloc+0x43/0x70
>> [ 6371.611457] __kmalloc+0x2b2/0x430
>> [ 6371.611459] __tty_buffer_request_room+0x100/0x250
>> [ 6371.611460] tty_insert_flip_string_fixed_flag+0x67/0x130
>> [ 6371.611462] pty_write+0xa2/0xf0
>> [ 6371.611463] n_tty_write+0x36b/0x7c0
>> [ 6371.611464] tty_write+0x275/0x500
>> [ 6371.611466] __vfs_write+0x50/0xa0
>> [ 6371.611467] vfs_write+0x10b/0x290
>> [ 6371.611468] redirected_tty_write+0x6a/0xc0
>> [ 6371.611470] do_iter_write+0x253/0x2b0
>> [ 6371.611471] vfs_writev+0x152/0x1f0
>> [ 6371.611472] do_writev+0xda/0x180
>> [ 6371.611474] __x64_sys_writev+0x45/0x50
>> [ 6371.611475] do_syscall_64+0xcc/0xaf0
>> [ 6371.611477] entry_SYSCALL_64_after_hwframe+0x49/0xb3
>>
>> [ 6371.611478] -> #2 (&port->lock#2){-.-.}-{2:2}:
>> [ 6371.611484] _raw_spin_lock_irqsave+0x3a/0x50
>> [ 6371.611486] tty_port_tty_get+0x22/0xa0
>> [ 6371.611487] tty_port_default_wakeup+0xf/0x30
>> [ 6371.611489] tty_port_tty_wakeup+0x39/0x40
>> [ 6371.611490] uart_write_wakeup+0x2a/0x40
>> [ 6371.611492] serial8250_tx_chars+0x22e/0x410
>> [ 6371.611493] serial8250_handle_irq.part.21+0x17c/0x180
>> [ 6371.611495] serial8250_default_handle_irq+0x5c/0x90
>> [ 6371.611496] serial8250_interrupt+0xa6/0x130
>> [ 6371.611498] __handle_irq_event_percpu+0x81/0x550
>> [ 6371.611499] handle_irq_event_percpu+0x70/0x100
>> [ 6371.611501] handle_irq_event+0x5a/0x8b
>> [ 6371.611502] handle_edge_irq+0x10c/0x370
>> [ 6371.611503] do_IRQ+0x9e/0x1d0
>> [ 6371.611505] ret_from_intr+0x0/0x37
>> [ 6371.611506] cpuidle_enter_state+0x148/0x910
>> [ 6371.611507] cpuidle_enter+0x41/0x70
>> [ 6371.611509] do_idle+0x3cf/0x440
>> [ 6371.611510] cpu_startup_entry+0x1d/0x1f
>> [ 6371.611511] start_secondary+0x29a/0x340
>> [ 6371.611513] secondary_startup_64+0xb6/0xc0
>>
>> [ 6371.611516] -> #1 (&port->lock){-.-.}-{2:2}:
>> [ 6371.611522] _raw_spin_lock_irqsave+0x3a/0x50
>> [ 6371.611525] serial8250_console_write+0x113/0x560
>> [ 6371.611527] univ8250_console_write+0x4b/0x60
>> [ 6371.611529] console_unlock+0x4e3/0x750
>> [ 6371.611530] vprintk_emit+0x10d/0x340
>> [ 6371.611532] vprintk_default+0x1f/0x30
>> [ 6371.611533] vprintk_func+0x44/0xd4
>> [ 6371.611535] printk+0x9f/0xc5
>> [ 6371.611537] register_console+0x262/0x3e0
>> [ 6371.611538] univ8250_console_init+0x23/0x2d
>> [ 6371.611540] console_init+0x268/0x395
>> [ 6371.611542] start_kernel+0x6c3/0x8b9
>> [ 6371.611544] x86_64_start_reservations+0x24/0x26
>> [ 6371.611546] x86_64_start_kernel+0xf4/0xfb
>> [ 6371.611548] secondary_startup_64+0xb6/0xc0
>>
>> [ 6371.611551] -> #0 (console_owner){-.-.}-{0:0}:
>> [ 6371.611558] __lock_acquire+0x21f8/0x3260
>> [ 6371.611560] lock_acquire+0x1a2/0x680
>> [ 6371.611562] console_unlock+0x2a2/0x750
>> [ 6371.611564] vprintk_emit+0x10d/0x340
>> [ 6371.611566] vprintk_default+0x1f/0x30
>> [ 6371.611568] vprintk_func+0x44/0xd4
>> [ 6371.611569] printk+0x9f/0xc5
>> [ 6371.611571] count_partial.cold.50+0x4d/0x52
>> [ 6371.611573] get_slabinfo+0x5c/0xb0
>> [ 6371.611575] dump_unreclaimable_slab.cold.35+0x97/0xe2
>> [ 6371.611577] dump_header+0x45a/0x510
>> [ 6371.611579] oom_kill_process+0xd0/0x280
>> [ 6371.611581] out_of_memory+0x478/0xa50
>> [ 6371.611583] __alloc_pages_slowpath.constprop.61+0x1680/0x1850
>> [ 6371.611585] __alloc_pages_nodemask+0x57c/0x6f0
>> [ 6371.611587] alloc_pages_vma+0x81/0x310
>> [ 6371.611589] do_anonymous_page+0x1bb/0x7a0
>> [ 6371.611591] __handle_mm_fault+0xbb0/0xbe0
>> [ 6371.611593] handle_mm_fault+0xdc/0x2e0
>> [ 6371.611595] do_page_fault+0x2cb/0x9d7
>> [ 6371.611597] page_fault+0x34/0x40
>>
>> [ 6371.611600] other info that might help us debug this:
>>
>> [ 6371.611603] Chain exists of:
>> [ 6371.611604] console_owner --> &port->lock#2 --> &n->list_lock
>>
>> [ 6371.611615] Possible unsafe locking scenario:
>>
>> [ 6371.611618] CPU0 CPU1
>> [ 6371.611619] ---- ----
>> [ 6371.611621] lock(&n->list_lock);
>> [ 6371.611625] lock(&port->lock#2);
>> [ 6371.611630] lock(&n->list_lock);
>> [ 6371.611634] lock(console_owner);
>>
>> [ 6371.611639] *** DEADLOCK ***
>>
>> [ 6371.611641] 5 locks held by oom02/43515:
>> [ 6371.611642] #0: ffff888ef72b4158 (&mm->mmap_sem#2){++++}-{3:3}, at: do_page_fault+0x1d6/0x9d7
>> [ 6371.611649] #1: ffffffff894dd268 (oom_lock){+.+.}-{3:3}, at: __alloc_pages_slowpath.constprop.61+0x90a/0x1850
>> [ 6371.611656] #2: ffffffff89520aa8 (slab_mutex){+.+.}-{3:3}, at: dump_unreclaimable_slab+0x2b/0x40
>> [ 6371.611661] #3: ffff8886456fcb98 (&n->list_lock){-.-.}-{2:2}, at: count_partial+0x29/0xe0
>> [ 6371.611668] #4: ffffffff893b8e60 (console_lock){+.+.}-{0:0}, at: vprintk_emit+0x100/0x340
>>
>> [ 6371.611675] stack backtrace:
>> [ 6371.611676] CPU: 1 PID: 43515 Comm: oom02 Not tainted 5.7.0-rc4-mm1+ #1
>> [ 6371.611679] Hardware name: HPE ProLiant DL385 Gen10/ProLiant DL385 Gen10, BIOS A40 07/10/2019
>> [ 6371.611680] Call Trace:
>> [ 6371.611681] dump_stack+0xa7/0xea
>> [ 6371.611682] print_circular_bug.cold.54+0x147/0x14c
>> [ 6371.611684] check_noncircular+0x295/0x2d0
>> [ 6371.611685] ? print_circular_bug+0x1d0/0x1d0
>> [ 6371.611686] ? __kasan_check_read+0x11/0x20
>> [ 6371.611688] ? mark_lock+0x160/0xfe0
>> [ 6371.611689] __lock_acquire+0x21f8/0x3260
>> [ 6371.611690] ? register_lock_class+0xb90/0xb90
>> [ 6371.611691] ? snprintf+0xc0/0xc0
>> [ 6371.611693] ? __kasan_check_read+0x11/0x20
>> [ 6371.611694] ? check_chain_key+0x1df/0x2e0
>> [ 6371.611695] lock_acquire+0x1a2/0x680
>> [ 6371.611697] ? console_unlock+0x240/0x750
>> [ 6371.611698] ? lock_downgrade+0x3e0/0x3e0
>> [ 6371.611699] ? check_flags.part.28+0x220/0x220
>> [ 6371.611701] ? rwlock_bug.part.1+0x60/0x60
>> [ 6371.611702] ? __kasan_check_read+0x11/0x20
>> [ 6371.611703] console_unlock+0x2a2/0x750
>> [ 6371.611705] ? console_unlock+0x240/0x750
>> [ 6371.611706] vprintk_emit+0x10d/0x340
>> [ 6371.611707] ? kernel_poison_pages.cold.3+0x86/0x86
>> [ 6371.611709] vprintk_default+0x1f/0x30
>> [ 6371.611710] vprintk_func+0x44/0xd4
>> [ 6371.611711] ? do_raw_spin_lock+0x11e/0x1e0
>> [ 6371.611712] printk+0x9f/0xc5
>> [ 6371.611714] ? log_store.cold.31+0x11/0x11
>> [ 6371.611715] ? count_partial+0x29/0xe0
>> [ 6371.611717] ? do_raw_spin_lock+0x11e/0x1e0
>> [ 6371.611718] count_partial.cold.50+0x4d/0x52
>> [ 6371.611719] get_slabinfo+0x5c/0xb0
>> [ 6371.611721] dump_unreclaimable_slab.cold.35+0x97/0xe2
>> [ 6371.611722] ? show_mem+0x10b/0x11c
>> [ 6371.611723] dump_header+0x45a/0x510
>> [ 6371.611724] oom_kill_process+0xd0/0x280
>> [ 6371.611726] out_of_memory+0x478/0xa50
>> [ 6371.611727] ? oom_killer_disable+0x230/0x230
>> [ 6371.611728] ? mutex_trylock+0x17a/0x190
>> [ 6371.611730] __alloc_pages_slowpath.constprop.61+0x1680/0x1850
>> [ 6371.611731] ? warn_alloc+0x120/0x120
>> [ 6371.611733] ? check_flags.part.28+0x220/0x220
>> [ 6371.611734] ? ___might_sleep+0x178/0x210
>> [ 6371.611735] ? __kasan_check_read+0x11/0x20
>> [ 6371.611737] __alloc_pages_nodemask+0x57c/0x6f0
>> [ 6371.611738] ? __alloc_pages_slowpath.constprop.61+0x1850/0x1850
>> [ 6371.611740] alloc_pages_vma+0x81/0x310
>> [ 6371.611741] do_anonymous_page+0x1bb/0x7a0
>> [ 6371.611742] ? __pte_alloc+0x170/0x170
>> [ 6371.611743] ? match_held_lock+0x35/0x270
>> [ 6371.611745] __handle_mm_fault+0xbb0/0xbe0
>> [ 6371.611746] ? copy_page_range+0x420/0x420
>> [ 6371.611747] ? sync_mm_rss+0x7f/0x190
>> [ 6371.611749] handle_mm_fault+0xdc/0x2e0
>> [ 6371.611750] do_page_fault+0x2cb/0x9d7
>> [ 6371.611751] page_fault+0x34/0x40
>>
>>
>>> ---
>>> mm/slub.c | 15 ++++++++++++++-
>>> 1 file changed, 14 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/slub.c b/mm/slub.c
>>> index 9bf44955c4f1..86a366f7acb6 100644
>>> --- a/mm/slub.c
>>> +++ b/mm/slub.c
>>> @@ -2407,16 +2407,29 @@ static inline unsigned long node_nr_objs(struct kmem_cache_node *n)
>>> #endif /* CONFIG_SLUB_DEBUG */
>>>
>>> #if defined(CONFIG_SLUB_DEBUG) || defined(CONFIG_SYSFS)
>>> +
>>> +static unsigned long max_partial_to_count __read_mostly = 10000;
>>> +module_param(max_partial_to_count, ulong, 0644);
>>> +
>>> static unsigned long count_partial(struct kmem_cache_node *n,
>>> int (*get_count)(struct page *))
>>> {
>>> + unsigned long counted = 0;
>>> unsigned long flags;
>>> unsigned long x = 0;
>>> struct page *page;
>>>
>>> spin_lock_irqsave(&n->list_lock, flags);
>>> - list_for_each_entry(page, &n->partial, slab_list)
>>> + list_for_each_entry(page, &n->partial, slab_list) {
>>> x += get_count(page);
>>> +
>>> + if (++counted > max_partial_to_count) {
>>> + pr_warn_once("SLUB: too much partial slabs to count all objects, increase max_partial_to_count.\n");
>>> + /* Approximate total count of objects */
>>> + x = mult_frac(x, n->nr_partial, counted);
>>> + break;
>>> + }
>>> + }
>>> spin_unlock_irqrestore(&n->list_lock, flags);
>>> return x;
>>> }
>