Subject: RE: kernel warning percpu ref in obj_cgroup_release

From: Christian Borntraeger <[email-masked]>
Date: Tue, 30 Mar 2021 17:10:36 +0200

On 30.03.21 15:49, Muchun Song wrote:
> On Tue, Mar 30, 2021 at 9:27 PM Christian Borntraeger
> <[email-masked]> wrote:
>>
>> So bisect shows this for belows warning:
> 
> Thanks for your effort on this. Can you share your config?

attached (but its s390x) for next-20210330

The problem goes away when I add
cgroup_controllers = [  ]
to /etc/libvirt/qemu.conf

The testcase that triggers the problem starts and stops multipe KVM guests with 248 CPUs.
Do we happen to have maybe only a byte of refcount space?

> 
>>
>> 636c3ef8229ecb4e7d045e86f36505d24a8f019a is the first bad commit
>> commit 636c3ef8229ecb4e7d045e86f36505d24a8f019a
>> Author: Muchun Song <[email-masked]>
>> Date:   Mon Mar 29 11:12:06 2021 +1100
>>
>>       mm: memcontrol: use obj_cgroup APIs to charge kmem pages
>>
>>       Since Roman's series "The new cgroup slab memory controller" applied.  All
>>       slab objects are charged via the new APIs of obj_cgroup.  The new APIs
>>       introduce a struct obj_cgroup to charge slab objects.  It prevents
>>       long-living objects from pinning the original memory cgroup in the memory.
>>       But there are still some corner objects (e.g.  allocations larger than
>>       order-1 page on SLUB) which are not charged via the new APIs.  Those
>>       objects (include the pages which are allocated from buddy allocator
>>       directly) are charged as kmem pages which still hold a reference to the
>>       memory cgroup.
>>
>>       We want to reuse the obj_cgroup APIs to charge the kmem pages.  If we do
>>       that, we should store an object cgroup pointer to page->memcg_data for the
>>       kmem pages.
>>
>>       Finally, page->memcg_data will have 3 different meanings.
>>
>>         1) For the slab pages, page->memcg_data points to an object cgroups
>>            vector.
>>
>>         2) For the kmem pages (exclude the slab pages), page->memcg_data
>>            points to an object cgroup.
>>
>>         3) For the user pages (e.g. the LRU pages), page->memcg_data points
>>            to a memory cgroup.
>>
>>       We do not change the behavior of page_memcg() and page_memcg_rcu().  They
>>       are also suitable for LRU pages and kmem pages.  Why?
>>
>>       Because memory allocations pinning memcgs for a long time - it exists at a
>>       larger scale and is causing recurring problems in the real world: page
>>       cache doesn't get reclaimed for a long time, or is used by the second,
>>       third, fourth, ...  instance of the same job that was restarted into a new
>>       cgroup every time.  Unreclaimable dying cgroups pile up, waste memory, and
>>       make page reclaim very inefficient.
>>
>>       We can convert LRU pages and most other raw memcg pins to the objcg
>>       direction to fix this problem, and then the page->memcg will always point
>>       to an object cgroup pointer.  At that time, LRU pages and kmem pages will
>>       be treated the same.  The implementation of page_memcg() will remove the
>>       kmem page check.
>>
>>       This patch aims to charge the kmem pages by using the new APIs of
>>       obj_cgroup.  Finally, the page->memcg_data of the kmem page points to an
>>       object cgroup.  We can use the __page_objcg() to get the object cgroup
>>       associated with a kmem page.  Or we can use page_memcg() to get the memory
>>       cgroup associated with a kmem page, but caller must ensure that the
>>       returned memcg won't be released (e.g.  acquire the rcu_read_lock or
>>       css_set_lock).
>>
>>       Link: https://lkml.kernel.org/r/[email-masked]
>>       Signed-off-by: Muchun Song <[email-masked]>
>>       Acked-by: Johannes Weiner <[email-masked]>
>>       Cc: Michal Hocko <[email-masked]>
>>       Cc: Roman Gushchin <[email-masked]>
>>       Cc: Shakeel Butt <[email-masked]>
>>       Cc: Vladimir Davydov <[email-masked]>
>>       Cc: Xiongchun Duan <[email-masked]>
>>       Signed-off-by: Andrew Morton <[email-masked]>
>>       Signed-off-by: Stephen Rothwell <[email-masked]>
>>
>>    include/linux/memcontrol.h | 116 +++++++++++++++++++++++++++++++++++----------
>>    mm/memcontrol.c            | 110 +++++++++++++++++++++---------------------
>>    2 files changed, 145 insertions(+), 81 deletions(-)
>>
>>
>>
>>
>>
>> On 30.03.21 13:32, Christian Borntraeger wrote:
>> [...]
>>>
>>> This next (328 is fine) triggers several bugs during our KVM CI run:
>>>
>>> [ 1506.494716] ------------[ cut here ]------------
>>> [ 1506.494730] percpu ref (obj_cgroup_release) <= 0 (-1) after switching to atomic
>>> [ 1506.494766] WARNING: CPU: 6 PID: 0 at lib/percpu-refcount.c:196 percpu_ref_switch_to_atomic_rcu+0x1ea/0x1f8
>>> [ 1506.494774] Modules linked in: kvm vhost_vsock vmw_vsock_virtio_transport_common vsock vhost vhost_iotlb xt_CHECKSUM xt_MASQUERADE xt_conntrack ipt_REJECT xt_tcpudp nft_compat nf_nat_tftp nft_objref nf_conntrack_tftp nft_counter bridge stp llc nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct dm_service_time nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 ip_set nf_tables nfnetlink zfcp scsi_transport_fc rpcrdma sunrpc dm_multipath rdma_ucm scsi_dh_rdac scsi_dh_emc rdma_cm scsi_dh_alua iw_cm ib_cm mlx5_ib ib_uverbs dm_mod ib_core s390_trng vfio_ccw vfio_mdev mdev vfio_iommu_type1 zcrypt_cex4 vfio eadm_sch sch_fq_codel configfs ip_tables x_tables ghash_s390 prng aes_s390 des_s390 libdes sha3_512_s390 sha3_256_s390 mlx5_core sha512_s390 sha256_s390 sha1_s390 sha_common nvme nvme_core pkey zcrypt rng_core autofs4 [last unloaded: vfio_ap]
>>> [ 1506.494832] CPU: 6 PID: 0 Comm: swapper/6 Not tainted 5.12.0-20210330.rc4.git0.9d49ed9ca93b.300.fc33.s390x+next #1
>>> [ 1506.494834] Hardware name: IBM 8561 T01 703 (LPAR)
>>> [ 1506.494836] Krnl PSW : 0704c00180000000 00000002d71dd21e (percpu_ref_switch_to_atomic_rcu+0x1ee/0x1f8)
>>> [ 1506.494840]            R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:0 PM:0 RI:0 EA:3
>>> [ 1506.494842] Krnl GPRS: c0000000fffeffff 00000002f7256818 0000000000000043 00000000fffeffff
>>> [ 1506.494844]            00000000ffffffea 0000038000000001 0000000000000000 000003800000017c
>>> [ 1506.494846]            00000002d7924988 0000000227eb97a0 000003ff5413c7e0 7fffffffffffffff
>>> [ 1506.494848]            0000000080360000 00000002f726b570 00000002d71dd21a 00000380000bba28
>>> [ 1506.494856] Krnl Code: 00000002d71dd20e: e3309fe8ff04        lg      %r3,-24(%r9)
>>>                             00000002d71dd214: c0e5001eb556        brasl   %r14,00000002d75b3cc0
>>>                            #00000002d71dd21a: af000000            mc      0,0
>>>                            >00000002d71dd21e: a7f4ffcc            brc     15,00000002d71dd1b6
>>>                             00000002d71dd222: 0707                bcr     0,%r7
>>>                             00000002d71dd224: 0707                bcr     0,%r7
>>>                             00000002d71dd226: 0707                bcr     0,%r7
>>>                             00000002d71dd228: eb6ff0480024        stmg    %r6,%r15,72(%r15)
>>> [ 1506.494928] Call Trace:
>>> [ 1506.494933]  [<00000002d71dd21e>] percpu_ref_switch_to_atomic_rcu+0x1ee/0x1f8
>>> [ 1506.494940] ([<00000002d71dd21a>] percpu_ref_switch_to_atomic_rcu+0x1ea/0x1f8)
>>> [ 1506.494942]  [<00000002d6b8a6c6>] rcu_do_batch+0x146/0x608
>>> [ 1506.494946]  [<00000002d6b8ec04>] rcu_core+0x124/0x1d0
>>> [ 1506.494948]  [<00000002d75d0222>] __do_softirq+0x13a/0x3c8
>>> [ 1506.494952]  [<00000002d6b05306>] irq_exit+0xce/0xf8
>>> [ 1506.494955]  [<00000002d75c1eb4>] do_ext_irq+0xdc/0x170
>>> [ 1506.494957]  [<00000002d75cdea4>] ext_int_handler+0xc4/0xf4
>>> [ 1506.494959]  [<0000000000000000>] 0x0
>>> [ 1506.494963]  [<00000002d75cd9c2>] default_idle_call+0x42/0x110
>>> [ 1506.494965]  [<00000002d6b411a0>] do_idle+0xd8/0x168
>>> [ 1506.494968]  [<00000002d6b413ee>] cpu_startup_entry+0x36/0x40
>>> [ 1506.494971]  [<00000002d6ac730a>] smp_start_secondary+0x82/0x88
>>> [ 1506.494974] Last Breaking-Event-Address:
>>> [ 1506.494975]  [<00000002d6b71898>] vprintk_emit+0xa8/0x110
>>> [ 1506.494978] Kernel panic - not syncing: panic_on_warn set ...
>>>
>>>
>>>
>>> I will try to bisect this, but if anyone has an idea. CC some candidates.
>>