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

[Konstantin Khlebnikov]

On 07/05/2020 06.01, Qian Cai wrote:
> > On May 6, 2020, at 3:06 PM, Qian Cai <cai@xxxxxx> wrote:
> >
> >
> >
> > > On May 4, 2020, at 12:07 PM, Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> >
> > 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;
> > > }