Date:   Mon, 26 Nov 2018 04:45:54 -0500
From:   Qian Cai <cai@....us>
To:     Waiman Long <longman@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>, arnd@...db.de,
        linux kernel <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH v4] debugobjects: scale the static pool size



On 11/25/18 11:52 PM, Qian Cai wrote:
> 
> 
> On 11/25/18 8:31 PM, Waiman Long wrote:
>> On 11/25/2018 03:42 PM, Qian Cai wrote:
>>>
>>>
>>> On 11/23/18 10:01 PM, Qian Cai wrote:
>>>>
>>>>
>>>>> On Nov 22, 2018, at 4:56 PM, Thomas Gleixner <tglx@...utronix.de>
>>>>> wrote:
>>>>>
>>>>> On Tue, 20 Nov 2018, Qian Cai wrote:
>>>>>
>>>>> Looking deeper at that.
>>>>>
>>>>>> diff --git a/lib/debugobjects.c b/lib/debugobjects.c
>>>>>> index 70935ed91125..140571aa483c 100644
>>>>>> --- a/lib/debugobjects.c
>>>>>> +++ b/lib/debugobjects.c
>>>>>> @@ -23,9 +23,81 @@
>>>>>> #define ODEBUG_HASH_BITS    14
>>>>>> #define ODEBUG_HASH_SIZE    (1 << ODEBUG_HASH_BITS)
>>>>>>
>>>>>> -#define ODEBUG_POOL_SIZE    1024
>>>>>> +#define ODEBUG_DEFAULT_POOL    512
>>>>>> #define ODEBUG_POOL_MIN_LEVEL    256
>>>>>>
>>>>>> +/*
>>>>>> + * Some debug objects are allocated during the early boot.
>>>>>> Enabling some options
>>>>>> + * like timers or workqueue objects may increase the size required
>>>>>> significantly
>>>>>> + * with large number of CPUs. For example (as today, 20 Nov. 2018),
>>>>>> + *
>>>>>> + * No. CPUs x 2 (worker pool) objects:
>>>>>> + *
>>>>>> + * start_kernel
>>>>>> + *   workqueue_init_early
>>>>>> + *     init_worker_pool
>>>>>> + *       init_timer_key
>>>>>> + *         debug_object_init
>>>>>> + *
>>>>>> + * No. CPUs objects (CONFIG_HIGH_RES_TIMERS):
>>>>>> + *
>>>>>> + * sched_init
>>>>>> + *   hrtick_rq_init
>>>>>> + *     hrtimer_init
>>>>>> + *
>>>>>> + * CONFIG_DEBUG_OBJECTS_WORK:
>>>>>> + * No. CPUs x 6 (workqueue) objects:
>>>>>> + *
>>>>>> + * workqueue_init_early
>>>>>> + *   alloc_workqueue
>>>>>> + *     __alloc_workqueue_key
>>>>>> + *       alloc_and_link_pwqs
>>>>>> + *         init_pwq
>>>>>> + *
>>>>>> + * Also, plus No. CPUs objects:
>>>>>> + *
>>>>>> + * perf_event_init
>>>>>> + *    __init_srcu_struct
>>>>>> + *      init_srcu_struct_fields
>>>>>> + *        init_srcu_struct_nodes
>>>>>> + *          __init_work
>>>>>
>>>>> None of the things are actually used or required _BEFORE_
>>>>> debug_objects_mem_init() is invoked.
>>>>>
>>>>> The reason why the call is at this place in start_kernel() is
>>>>> historical. It's because back in the days when debugobjects were
>>>>> added the
>>>>> memory allocator was enabled way later than today. So we can just
>>>>> move the
>>>>> debug_objects_mem_init() call right before sched_init() I think.
>>>>
>>>> Well, now that kmemleak_init() seems complains that
>>>> debug_objects_mem_init()
>>>> is called before it.
>>>>
>>>> [    0.078805] kmemleak: Cannot insert 0xc000000dff930000 into the
>>>> object search tree (overlaps existing)
>>>> [    0.078860] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.20.0-rc3+ #3
>>>> [    0.078883] Call Trace:
>>>> [    0.078904] [c000000001c8fcd0] [c000000000c96b34]
>>>> dump_stack+0xe8/0x164 (unreliable)
>>>> [    0.078935] [c000000001c8fd20] [c000000000486e84]
>>>> create_object+0x344/0x380
>>>> [    0.078962] [c000000001c8fde0] [c000000000489544]
>>>> early_alloc+0x108/0x1f8
>>>> [    0.078989] [c000000001c8fe20] [c00000000109738c]
>>>> kmemleak_init+0x1d8/0x3d4
>>>> [    0.079016] [c000000001c8ff00] [c000000001054028]
>>>> start_kernel+0x5c0/0x6f8
>>>> [    0.079043] [c000000001c8ff90] [c00000000000ae7c]
>>>> start_here_common+0x1c/0x520
>>>> [    0.079070] kmemleak: Kernel memory leak detector disabled
>>>> [    0.079091] kmemleak: Object 0xc000000ffd587b68 (size 40):
>>>> [    0.079112] kmemleak:   comm "swapper/0", pid 0, jiffies 4294937299
>>>> [    0.079135] kmemleak:   min_count = -1
>>>> [    0.079153] kmemleak:   count = 0
>>>> [    0.079170] kmemleak:   flags = 0x5
>>>> [    0.079188] kmemleak:   checksum = 0
>>>> [    0.079206] kmemleak:   backtrace:
>>>> [    0.079227]      __debug_object_init+0x688/0x700
>>>> [    0.079250]      debug_object_activate+0x1e0/0x350
>>>> [    0.079272]      __call_rcu+0x60/0x430
>>>> [    0.079292]      put_object+0x60/0x80
>>>> [    0.079311]      kmemleak_init+0x2cc/0x3d4
>>>> [    0.079331]      start_kernel+0x5c0/0x6f8
>>>> [    0.079351]      start_here_common+0x1c/0x520
>>>> [    0.079380] kmemleak: Early log backtrace:
>>>> [    0.079399]    memblock_alloc_try_nid_raw+0x90/0xcc
>>>> [    0.079421]    sparse_init_nid+0x144/0x51c
>>>> [    0.079440]    sparse_init+0x1a0/0x238
>>>> [    0.079459]    initmem_init+0x1d8/0x25c
>>>> [    0.079498]    setup_arch+0x3e0/0x464
>>>> [    0.079517]    start_kernel+0xa4/0x6f8
>>>> [    0.079536]    start_here_common+0x1c/0x520
>>>>
>>>
>>> So this is an chicken-egg problem. Debug objects need kmemleak_init()
>>> first, so it can make use of kmemleak_ignore() for all debug objects
>>> in order to avoid the overlapping like the above.
>>>
>>> while (obj_pool_free < debug_objects_pool_min_level) {
>>>
>>>      new = kmem_cache_zalloc(obj_cache, gfp);
>>>      if (!new)
>>>          return;
>>>
>>>      kmemleak_ignore(new);
>>>
>>> However, there seems no way to move kmemleak_init() together this
>>> early in start_kernel() just before vmalloc_init() [1] because it
>>> looks like it depends on things like workqueue
>>> (schedule_work(&cleanup_work)) and rcu. Hence, it needs to be after
>>> workqueue_init_early() and rcu_init()
>>>
>>> Given that, maybe the best outcome is to stick to the alternative
>>> approach that works [1] rather messing up with the order of
>>> debug_objects_mem_init() in start_kernel() which seems tricky. What do
>>> you think?
>>>
>>> [1] https://goo.gl/18N78g
>>> [2] https://goo.gl/My6ig6
>>
>> Could you move kmemleak_init() and debug_objects_mem_init() as far up as
>> possible, like before the hrtimer_init() to at least make static count
>> calculation as simple as possible?
>>
> 
> Well, there is only 2 x NR_CPUS difference after moved both calls just after 
> rcu_init().
> 
>           Before After
> 64-CPU:  1114   974
> 160-CPU: 2774   2429
> 256-CPU: 3853   4378
> 
> I suppose it is possible that the timers only need the scale factor 5 instead of 
> 10. However, it needs to be retested for all the configurations to be sure, and 
> likely need to remove all irqs calls in kmemleak_init() and subroutines because 
> it is now called with irq disabled. Given the initdata will be freed anyway, 
> does it really worth doing?
> 
> BTW, calling debug_objects_mem_init() before kmemleak_init() actually could 
> trigger a loop on machines with 160+ CPUs until the pool is filled up,
> 
> debug_objects_pool_min_level += num_possible_cpus() * 4;
> 
> [1] while (obj_pool_free < debug_objects_pool_min_level)
> 
> kmemleak_init
>    kmemleak_ignore (from replaced static debug objects)
>      make_black_object
>        put_object
>          __call_rcu (kernel/rcu/tree.c)
>            debug_rcu_head_queue
>              debug_object_activate
>                debug_object_init
>                  fill_pool
>                    kmemleak_ignore (looping in [1])
>                      make_black_object
>                        ...
> 
> I think until this is resolved, there is no way to move debug_objects_mem_init() 
> before kmemleak_init().

I believe this is a separate issue that kmemleak is broken with 
CONFIG_DEBUG_OBJECTS_RCU_HEAD anyway where the infinite loop above could be 
triggered in the existing code as well, i.e., once the pool need be refilled 
(fill_pool()) after the system boot up, debug object creation will call 
kmemleak_ignore() and it will create a new rcu debug_object_init(), and then it 
will call fill_pool() again and again. As the results, the system is locking up 
during kernel compilations.

Hence, I'll send out a patch for debug objects with large CPUs anyway and deal 
with kmemleak + CONFIG_DEBUG_OBJECTS_RCU_HEAD issue later.

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